Turn off the water valves at the main incoming water pipe. This is normally near your water heater. Then open the shower faucet by turning the handle to on and drain excess water in the pipes.
Remove the cap on the faucet handle and loosen the screw underneath with an Allen wrench. Pull the handle straight back and off of the faucet.
Pry out the clip around the cartridge with a pair of needle-nose pliers. Then pull the cartridge out of the faucet head by hand or with a pair of pliers. Note that if there are additional retaining screws on top of the cartridge, you should remove them first with channel pliers.
Install a new cartridge inside the faucet head. Simply slip it inside the head. Then replace the retaining clip and additional securing hardware if necessary.
Put the handle back on the faucet stem and install the set screw. Tighten this screw with an Allen wrench to complete the installation.
Turn on the main water valve and operate the faucet to test it and check for leaks.