If the floor tiles are very uneven, you may decide to replace them or re-cover the floor; a self-smoothing floor screeding compound is applied directly over the tiles to form an ideal sub-floor. Cracked or crazed tiles cannot be repaired; they must be replaced with new ones. You can, however, make a temporary repair on slightly pitted tiles with cementwork or adhesive filler. Remove the damaged tile carefully so you do not damage adjacent tiles.
You should wear protective spectacles when breaking up the tile with a hammer and small cold chisel; start at the centre of the tile and work outwards. When the tile has been removed piece by piece, chip away the bedding mortar so the new tile will lie just below the surface of the existing tiles. Coat the back of the new tile with a thin layer of floor tile adhesive, using a notched spreader. Press the tile into place so it is level with adjacent tiles and move it so there is an equal gap around all its edges. After 24 hours you can fill the joints around the tile with grout or a mortar mix consisting of four parts fine, sharp sand to one part Portland cement.