Open the panel behind the tub wall.and turn the valves to the left to shut off the water to the faucet.
Pull off the cap on the faucet handle. Loosen the small screw inside the handle with the Allen wrench until you are able to slide the handle straight off of the fixture.
Pry out the round, retaining clip from the edge of the faucet cartridge using pliers.
Loosen the retaining washer, if there is one, with the pliers. This is a round nut that fits over the head of the faucet. Slide the cartridge off the faucet head by pulling it back and out of the faucet head.
Insert a new, matching cartridge into the faucet head. Replace the washer and retainer clip.
Put the handle back on the cartridge stem. Insert the screw in the faucet handle and tighten it with the Allen wrench until secure. Put the cover on the handle by sliding it over the stem, pressing it into place by hand