I am renovating a bathroom and want to change the tiles in the shower. I was wondering if I could install the new tiles overtop the existing tiles to avoid destroying the wall and what steps to follow if this can be done. The existing tile is in great shape but not the desired colour.

I think you're going to have a lot of trouble trying to get new tiles to adhere to old tiles. When it fails (and it will), you'll have to throw away the newer tiles and start over. You'll be farther ahead to remove the old tiles.

I agree, you should tear out the old tiles. Double tickness with cause trouble with the handles and escutcheon plates and you never know if the old tile was installed in the correct fashion and will last another 20 years. 20 years ago Hardiebackre, Duroroick, Wonderboard and Shluter Ditra wrere not invented and that's what we all use now under a shower tile wall.