How much time would it take to re-tile?
So you first have to remove the old tiles, with a hammer and chisel, as suggested. But, that is a point of no return!
Then you need to assess the wall surface under the tiles, and what action is needed, it might need re-plastering.
Then, you need to re-tile. Tiling isn't too difficult if the wall is flat, but you need to be a hands-on type of person.
Google all this, it's all on there.