Quran 6:34 states:
لَا مُبَدِّلَ لِكَلِمَـٰتِ ٱللَّهِ ۚ
There is none that can alter the words of Allah.
Quran 6:115 states:
وَعَدْلًۭا ۚ لَّا مُبَدِّلَ لِكَلِمَـٰتِهِۦ ۚ وَهُوَ ٱلسَّمِيعُ ٱلْعَلِيمُ
None can alter His words, and He is the Hearing, the Knowing.
Quran 18:27 states:
وَٱتْلُ مَآ أُوحِىَ إِلَيْكَ مِن كِتَابِ رَبِّكَ ۖ لَا مُبَدِّلَ لِكَلِمَـٰتِهِۦ وَلَن تَجِدَ مِن دُونِهِۦ مُلْتَحَدًۭا ٢٧
And recite, what has been revealed to you of the Book of your Lord. There is no changer of His words, and never will you find in other than Him a refuge.
Since Bible is also considered “word of Allah” by Muslims, these ayats clearly refutes the common notion that older Holy Books have changed.
Tafsir Ibne Kathir of Quran 3:78 States:
Al-Bukhari reported that Ibn `Abbas said that the Ayah means they alter and add although none among Allah’s creation can remove the Words of Allah from His Books, they alter and distort their apparent meanings. Wahb bin Munabbih said, “The Tawrah and the Injil remain as Allah revealed them, and no letter in them was removed. However, the people misguide others by addition and false interpretation, relying on books that they wrote themselves.