Turn off the water supply to the shower
Open the shower valve to relieve pressure
Remove the shower handle
Remove the trim plate and any screws
Access the valve behind the wall
Disconnect the water lines from the old valve
Remove the old shower valve
Install the new shower valve
Connect the water lines to the new valve
Secure the valve in place
Reinstall the trim plate and handle
Turn the water supply back on
Test for leaks and proper operation