Turn off the shower's main water supply at the shutoff valve located next to the water meter.
Remove the faucet's handle. Pry off the decorative cap located at the center of the faucet's handle with the blade of a flat-head screwdriver. Remove the handle's retaining screw with a Phillips-head screwdriver, then pull the handle away from the faucet valve.
Remove the plate that rests against the shower wall behind the faucet's handle. Unscrew the plate's retaining screw, then pull the plate away from the wall.
Unscrew the faucet valve's retaining nut. Insert a shower stem socket through the shower wall onto the nut and turn it counterclockwise.
Grasp the tip of the shower cartridge with a pair of pliers, then pull the cartridge straight out of the wall. Insert the replacement cartridge into the wall and tighten the valve's retaining nut with the shower stem socket.
Place the valve's plate against the wall. Tighten the plate's retaining screw with a screwdriver.
Slide the faucet's handle over the faucet. Tighten the handle's retaining screw through the center of the handle. Press the handle's decorative cap over the handle's retaining screw. Turn on the water supply to complete the replacement.