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.
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.