Ceramic floor’d can look grungy in time, crumbly and in need of replacement. Chances are the tile is fine (it can last nearly forever), and it’s just the grout that needs to be replaced. It is usually not necessary to remove all the grout from the floor; remove just the parts that are damaged, loose or mildewed beyond cleaning.
Use a grout saw (it looks like a thick razor blade on the end of a handle) to dig out any loose or crumbling grout. Work slowly, getting it out little by little rather than trying to remove it all at once. Tap gently with the hammer and chisel to loosen grout that isn't crumbling but needs to be replaced because of mildew or discoloration. Then use the grout saw to remove that grout. Use a vacuum hose to thoroughly clear the loose grout from the spaces between the tiles.
Mix powdered grout in a bucket until it's the consistency of cake frosting. Let it sit in the bucket for 10 minutes. Use a grout float (a rubber trowel) to spread fresh grout onto areas of the floor where the old grout has been removed. Press it into the spaces between the tiles while using the edge of the float to swipe the excess off the tile face. Let it sit in the lines for a minute, then wipe the surface with a damp sponge to smooth the grout and remove any excess.
Let the new grout set for two days.
Fold a piece of sandpaper to create an edge as thick as the grout lines. Buff the surface of the grout lines with the sandpaper, sanding both new and old grout, to bring them to a uniform smoothness. Use a vacuum hose to remove the resultant grout dust.
Apply grout sealer over all the grout lines, using a thin brush.