Alleged Contradictions in the Qur'an answer
Abrogation? "The words of the Lord are perfect in truth and justice; there is NONE who can change His words." [Sura 6:115] Also see 6:34 and 10:65. But then Allah (Muhammad?) sees the need to exchange some of them for "better ones" [Sura 2:106, 16:101].

Verses in question:
6:115. And the Word of your Lord has been fulfilled in truth and in justice. None can change His Words. And He is the All Hearer, the All Knower.

2:106. Whatever a Verse (revelation) do We abrogate or cause to be forgotten, We bring a better one or similar to it. Know you not that Allâh is able to do all things?

16:101. And when We change a Verse in place of another, and Allâh knows the best of what He sends down, they (the disbelievers) say: "You (O Muhammad ) are but a forger!" Nay, but most of them know not.
1. This objection is based on a misinterpretation of verse 6:115, hence we shall first provide the correct explanation of this verse before moving on to explain the concept of abrogation in the Qur'an. With regards to verse 6:115 and all similar statements in the Qur'an, the phrase 'none can change His words' does not necessarily refer to literal verbal revelation. Nor does the suspension of a verse's ruling necessarily equate the changing of God's words. What is understood from this verse and similar verses in the Qur'an is that the 'words' being referred to here are the decrees of Allah. Let us examine what Imam Ibn Kathir Ad-Damishqi (d. 1372CE) says regarding this phrase in his Tafsir Al-Qur'an Al-Azim:
(and none can alter the Words of Allah.) This refers to His decision that victory in this life and the Hereafter is for His believing servants. Allah said in other Ayat,
(And, verily, Our Word has gone forth of old for Our servants, the Messengers. That they verily would be made triumphant. And that Our hosts, they verily would be the victors.) (Tafsir Ibn Kathir, Abridged, Darussalam Publishers & Distributors 2000, vol. 3, pp. 338-339)

(None can change His Words.) meaning, none can avert Allah's judgment whether in this life or the Hereafter, (Tafsir Ibn Kathir, Abridged, Darussalam Publishers & Distributors 2000, vol. 3, p. 447)
Thus, we can see that for both verses 6:34 and 6:115, Ibn Kathir has explained that this phrase simply relates to the unalterable decrees and laws of Allah. Our destiny in this life has already been dictated by Allah and recorded in the Preserved Tablet (Al-Lawh Al-Mahfudh). This tablet contains the words of Our Creator, and these decrees cannot be changed. Abul Ala Maududi (d. 1979 CE) expresses this concept in the following terms in his commentary on verse 6:34:
The point emphasized here is that no one has the power to change God's Law regarding the conflict between Truth and falsehood. Lovers of Truth must of necessity pass through trials and persecutions so as to be gradually tempered. Their endurance, their honesty of conviction, their readiness to sacrifice and to undertake all risks for their cause, the strength of their faith and the extent of their trust in God must be tested. They must pass through this phase of persecution to develop in themselves those qualities which can be developed nowhere else but on earth. They are also required to defeat the forces of Ignorance by virtue of their moral excellence and the nobility of their character. Only after they have established their moral superiority over the adversaries will God's help arrive. No one can secure that help beforehand. (Maududi, Towards Understanding the Qur'an, The Islamic Foundation 1995, vol. 2, p.227 fn. 22)
Hence, Maududi emphasizes that the decree is associated with the law relating to our position and task in this wordly life. Moiz Amjad presents a similar argument in his article, [Link only for registered members] , in which he writes:
I have explained the three referred verses and have shown that if interpreted in the light of their respective contexts, none of these verses can be taken to refer to the books revealed by God. In the first of the three verses (Al-An`aam 6: 34) 'the words of God' refer to the God's law regarding the rejection of His messengers. In the second verse (Al-An`aam 6: 115) the phrase 'the word of your Lord' refers to the unchanging law of God regarding who is allowed to accept His guidance and who is not. Finally, in the third verse (Yunus 10: 64), 'the words of God' refer to the unalterable law of God regarding reward of the pious. None of the referred verses, as evidenced by their context, refers to the books or verses revealed by God. They refer to certain moral laws of God, which the Qur'an has declared to be God's constants

By bringing together the mentioned points we may arrive at the understanding that the words referred to in these verses are those that dictate the fate of the universe. While the decrees of Allah may be presented in the verses of the Qur'an, the nullification of certain rulings of these verses does not equate the changing of Allah's decrees, as shall be elaborated under the next point.

2. With regards to abrogation (Ar. naskh), it is a confirmed Islamic doctrine in the Qur'an. Since the Qur'an was revealed gradually over a period of twenty-three years, the legal rulings were not imposed on its adherents all at once. Rather, it gave them time to grow in faith and become accustomed to Islam. As Shaykh Abu Ammaar Yasir Qadhi mentions:
Among the blessings of Allaah to the Companions is that He revealed to them the laws of Islaam gradually, and thus made it easier for them to adopt these laws. Initially, there were no specific laws of halaal and haraam. The Companions during the Makkan stage were being trained spiritually so that they could form the nucleus of the future Muslim state in Madeenah. Once they had passed this stage, Allaah then completed the revelation of the sharee'ah in gradual steps, so that they could adapt to the lifestyle of Islaam.(Qadhi, An Introduction to the Sciences of the Qur'aan, Al-Hidaayah Publishing and Distribution 1999, p. 86)
And as Makkee ibn Abee Taalib (d. 1035CE) mentioned regarding abrogation:
And this (meaning naskh) is from Allah, and is meant to be for the betterment of His worshippers. So, He commands them with a ruling at a specific time, since He knows that it will be for their betterment for that particular time, but He already knows that this command will be removed from them at a later time, since at this later time that particular ruling will not be for their benefit. (An-Nahaas, p. 116.)
Thus, abrogation does not imply any imperfection whatsoever on the part of God, as critics allege. It does not mean that God made a mistake or that he didn't foresee an event. Rather, God knew in advance, and intended to send temporary laws for the early Muslims that would later be abrogated once the Muslim society became established.
It is analogous to a Professor who asks his students to perform 30 minutes of studying everyday for the first week. During the second week, he 'abrogates' his initial command and asks his students to perform 1 hour of studying every day. The Professor did not make a mistake initially, nor did he react to an unforeseen event. Rather, he had always planned to give a lighter load the first week to his students, and then increase the workload the next week because he knew they would be ready for it. In fact, he had his plan for the entire course written down and recorded. So when he initially gave the order to perform 30 minutes of homework, he knew that he would later abrogate this command.
Similarly, Allah initially gave some rulings that were later abrogated, but He knew and intended for them to be abrogated as the condition of the Muslims changed. He also recorded all His decrees in the Preserved Tablet; everythign was planned in advance. One may ask why some narrations seem to imply that God's revelation was a reaction to an event. The answer is that God revealed the laws at the occurence of such events so that the Muslims could appreciate the practical application of the law, and the need for such a law.

3. So the question that remains is, "Does the doctrine of abrogation contradict the verses stating that none can change the decrees of God?" And the answer is no, because God had decreed right from the beginning that He would reveal temporary laws that would be abrogated. He decreed right from the beginning that the Muslims would follow this law for this amount of time, and then later it would be abrogated by this law. That was His decree, His word. Abrogation is not the changing of His words, but it is the addition of a new ruling to be followed, as originally decreed. Going back to the example of the Professor, we could say that:
The Professor told his class at the beginning of the course, "I've written down what we will do during this course. I've planned it carefully, and written down everything we will do, word for word. No one can change my words." Then, during the second week when the Professor changes the ruling from 30 minutes of homework to 1 hour, a student complains and says, "But sir, you've changed your words!" The Professor responds by saying, "No, I haven't changed my words, I had planned this right from the beginning." Evidently, he is reffering to his plan as his words.
Thus, Allah has informed us that no one can alter the fate which He has decreed for the universe, and no one can repeal His decisions and decrees. This is completely different from the phenomenon of naskh, in which a new ruling is followed, in accordance with His original decree. Consequently, there is no contradiction between the two verses.