Turn off the water supply to the shower
Open the faucet to drain any remaining water
Remove the shower handle
Remove the trim plate and any decorative covers
Inspect the valve type and take a photo for reference
Cut or disconnect access to the valve from the wall if needed
Disconnect the supply lines or pipes from the old valve
Remove the old valve body from the plumbing
Install the new valve in the same orientation
Secure the new valve to the framing or pipe supports
Reconnect the water supply lines or pipes
Install the trim plate and shower handle
Turn the water supply back on
Check for leaks at all connections
Test hot and cold water operation
Seal around the trim plate if needed