Shut the incoming water valves. Pry off the cap on the handle of the shower faucet with a small knife and unscrew the Allen nut beneath it with an Allen wrench.
Slide the handle off and then remove the clip around the cartridge. This is a metal ring attached to the cartridge right under the handle. Pry it off with a pair of pliers or a screwdriver.
Pull the cartridge out of the faucet head by hand. Or, use a cartridge extractor to remove the cartridge if hard water and deposits have made it difficult to remove. Normally, this is not necessary.
Remove the seat springs inside the faucet head with the tip of the screwdriver if they did not come out with the cartridge. Insert the screwdriver into the two valve holes and just lift them out.
Put a new cartridge into the head of the faucet and slide it all the way back inside.
Reinstall the clip and replace the handle. Tighten the Allen nut to secure the handle in place. Turn the water on and test the faucet.