Turn the water off by closing the home's water main at the water heater.
Loosen the screw in the base of the handle with an Allen wrench and pull back on the handle to slide it off the faucet. Pop the trim plate off the wall by hand. The valve is underneath the handle and trim.
Unscrew the retainer screws inside the shower wall on either side of the faucet valve retainer. This is the part that holds the valve to the shower body. Use a screwdriver to unscrew both screws and pull back on the retainer to remove it.
Place a new retainer and valve on the body of the shower and screw the two screws in with a screwdriver. Replace the trim plate and handle. Tighten up the handle screw to secure it to the faucet.