Many Christians would say the Bible was the unchanging constant word of God, especially if you went back in time a bit. You might argue they aren't your kind of Christian but all that does is prove that people are allowed to have differences of opinion. Religions change as people change. For example in North Korea they are ruled by a God/cult right now in the present day, but at some point they won't be. Things change.The Koran, unlike JC who may be interpreted and updated as a moving icon , the Koran is literal, quite deliberately , and Muhammad its receiver is supposed to be the final prophet.
The Koran can be interpreted with understanding of Muhammad as a man of his place and time. I cannot understand the Muslims' huge adulation of Muhammad which seems indistinguishable from idolatry.
Also to understand Muslims' adulation you only need to understand your own adulation, there is no difference, Muslims (and Koreans) are people too.