Replacement 1/4 turn cartridges are available but i suggest taking old cartridge along for comparison.First remove the tap lever by either giving it a good tug to remove or locate fixing screw in lever and undo(could be a screw under the hot/cold ID tags or a little hex grub screw).Next obtain a good size spanner that is a snug fit on the cartridge hex and undo as any normal nut,ie,anti clockwise,(may be very tight),be sure to take measures to prevent tap body moving then just put it all together the opposite way from removal.
steffdaplum
October 2009