Locate the water line in your home and turn the valve to shut off the water flow.
Remove the fixture from your bathroom wall. The many different fixtures can come off in a variety of ways. However, start by popping the "H" and "C" labeled covers off the shower or sink handles. This will likely expose screws underneath. Remove the screws with your screwdriver.
Unscrew the handles to expose a metal knob.
Remove any screws on the metal knob.
Grip the metal knob with a plumber's wrench and pull it off. You will now have access to the guard covering the valve.
Turn the guard counterclockwise to unscrew it and slide it off the valve.
Slide a new guard in the old one's place and screw it in.
Replace the metal knob and handles.