The words of the Lord are perfect in truth and justice;
there is NONE who can change His words.
He both heareth and knoweth.
-- Sura 6:115
None can change the words of God;
-- Sura 6:34
There is no changing the words of God;
that is the mighty triumph.
-- Sura 10:65
And recite what has been revealed to you of the Book of your Lord,
there is none who can alter His words;
and you shall not find any refuge besides Him.
-- Sura 18:27
However:
And for whatever verse we abrogate and cast into oblivion
We bring a better or the like of it;
knowest thou not that God is powerful over everything?
-- Sura 2:106
And when We exchange a verse in place of another verse --
and God knows very well what He is sending down --
they say, 'Thou art a mere forger!'
Nay, but the most of them have no knowledge.
-- Sura 16:101
The problem is obvious: On the one hand, Sura 6:115, 6:34, 10:65, 18:27 make it clear that NONE CAN change the words of God, but, on the other hand, God DOES exchange one verse for another verse (Sura 2:106, 16:101). And he does so through his messengers like Jesus [supposedly changing some rules given through Moses] and like Muhammad who gives rules different again from those of Moses and of Jesus.