Turn off the water supply to the shower
Open the shower valve to drain any remaining water
Remove the shower handle and trim plate
Remove the valve cartridge or stem
Access the valve body from the wall or behind the plumbing access panel
Disconnect the old valve from the water lines
Remove the old valve from the wall
Install the new valve in the same orientation
Connect the hot and cold water lines to the new valve
Secure the valve in place
Install the new cartridge or stem
Reattach the trim plate and handle
Turn the water supply back on
Check for leaks
Test the shower for proper hot and cold operation