If you damage the wall significantly you have to repair it before installing the new tile.
Scrape tile adhesive off wall.
Do not gouge the walls.
Usually you can get a few bits of the adhesive off from scraping the tiles.
If the glue is thick you may need to reapply.
The first step is to use a putty knife to scrape off the adhesive while applying heat to the adhesive to soften it up.
Step 3 wipe some tile adhesive remover over the surface of the wall.
Adding water will damage the wallpaper.
Scrape off the adhesive.
If only traces of the glue remain scrub very gently with a dry scotch scrubbing pad.
Scrape down the walls with a putty knife removing the softened adhesive.
If so repeat steps 3 5 as necessary.
The critical aspect is to scrape the adhesive off the wall surface as much as possible by using a wide chisel and a hammer.
By working with patience and with great care you will remove the largest amount of the wall tile adhesive.
How to easily remove old tile adhesive and grout from walls using a heavy duty scraper uploaded by upload to youtube for windows phone.
This may take a while depending on the size of the wall and the amount of adhesive you need to get rid of.
If any adhesive gets stuck to the brush remove it by hand and continue until no more adhesive remains on the wall.
When the adhesive is softened through and can be easily cut through with a putty knife it is ready to come off.
Scrape the surface of the adhesive with a stiff bristle brush.
Once the mortar softens up scrape it off with a 4 scraper or a wallpaper stripper.
Use the straight edge or razor blade to gently scrape the glue off.
Repeat the process until all tile adhesive is removed.
If the adhesive has dried you will need to either scrape it off use the apropriate thinner or sometimes it will simply peel off.
It depends on the adhesive some are also water based and clean off with warm water.