Muslims say that a prerogative of the Quran is abrogation, which involves the annulling of a former law by a new law. Sura 16:101 says, "When We substitute one revelation for another, and God knows best what He reveals (in stages), they say, 'Thou art but a forger': but most of them understand not."
What this means is that Allah is not bound to his revelations. If he wants, he is free to bring new revelation that completely contradicts former revelation. If circumstances call for it, Allah is free to rescind earlier revelations and bring about something entirely new and different.
An example of such a change is that, originally, Muhammad ordered his followers to pray toward Jerusalem (Sura 2:150). However, when the Jews rejected him and called him an impostor, he received new revelation to the effect that the correct direction of prayer should be toward Mecca (Sara 2:125). This change is in keeping with what we read in Sura 2:106: "If we abrogate a verse or consign it to oblivion, we offer something better than it or something of equal value."
Islamic scholars do not agree among themselves as to the precise number of verses that have become abrogated by other revelations. But there are at least 20 instances in which a newer revelation has been said to supersede, contradict, or abrogate a previous revelation." All this has given rise to a Quranic science known as Nasikh wa Mansukh, that is, "the Abrogators and the Abrogated."1
1. Excerpt from Ron Rhodes, Reasoning from the Scriptures with Muslims (pp. 75-76).