One of the quickest ways to give a face lift to sound old tiling is to replace the grout filling material round each tile. Rake out the old grout. Take care not to scratch surface of tiles while raking out. Remove broken tiles and old adhesive with small sharp cold chisel and club hammer.
Work from centre outwards and take care as You get near surrounding tiles; protect any fittings from being scratched by broken pieces of tile. Always wear Protective spectacles or goggles.
Spread new grout over tiles with dampened sponge. Remove any surplus material as you work, leaving just enough to fill gaps between tiles. When grout is almost dry, clean tiles with dampened sponge. Finish off with clean, dry soft cloth.