The mixer faucets used in showers feature essentially the same design as those mixers used in kitchens, but the process of accessing the faucet valve differs slightly. This faucet design derives its name from a series of passageways that change alignment when the handle is operated to combine or "mix" hot and cold water to the desired temperature. Replacement mixer valves are fairly inexpensive, and replacing one is relatively straightforward.
Turn the water supply off at the main shutoff valve with a water main wrench.
Pry off the plastic trim plate from the middle of the handle with a screwdriver to reveal the handle's securing screw.
Remove the handle's securing screw from the handle with a screw driver to release the handle from the tip of the faucet.
Pull the metal collar that surrounds the faucet out of the shower wall to reveal a retaining clip at the top of the valve.
Pull the retaining clip out of the valve with pliers.
Pull the faucet valve out of the wall.
Insert the new faucet valve into the wall.
Slide the retaining clip into the top of the valve.
Slide the metal collar over the valve and into the wall.
Place the handle onto the tip of the valve and tighten the securing screw into the handle with the screwdriver.
Press the plastic trim plate into the handle until it clicks into place.