If any of your shower tiles have cracks in them or are missing grout, water can get behind the tiles, causing them to buckle, crack further or fall off the walls. The more the tiles are damaged, the more water can get behind the remaining good tiles. If you can find new tiles to match the damaged ones, you can extract the damaged ones and replace them without having to redo the whole shower enclosure.
Use a grout saw to dig out the grout on all four sides of each damaged tile, breaking it up and blowing away the debris.
Hold a masonry chisel into the open line of one of the damaged tiles and tap at the base of the tile, hitting the chisel gently with your hammer. Don't try to pry it out, but work your way around the whole base of each tile, chipping the mortar until the tile pops out.
Remove all of the damaged tiles. Scrape off any remaining mortar from the wall using a chisel and hammer. If the wall is at all wet from the damage, let it dry completely.
Cover the back of a new tile with thin-set mortar, applying it with the notched side of a mortar trowel and making it about 1/8 inch thick. Press the tile to the wall, spacing evenly with the existing tiles. Do the same for each new tile.
Let the mortar set overnight.
Spread grout over the newly tiled area with a rubber grout trowel, pressing it into the spaces while scraping it off the surface. Wipe up the excess grout with a damp sponge. Let the grout set for two to three days.
Wear eye protection when removing the tiles.