Karl Hockless
Smash old one with hammer and chisel. Wear goggles and gloves! Scrape out old adhesive and grout without damaging neighbouring tiles. Prime with 3 to 1 SBR bond and water. Set new tile with 1 part rapid flexible tile adhesive. Clean off excess adhesive. Grout with flexible grout when the adhesive has set. To do this properly as detailed above will probably cost you around £50 in materials of which you will have lots left over. I would try and get a tiler to do it, it will probably cost around the same. Cheers, Karl