Turn off the water supply to the shower
Open the shower faucet to drain remaining water
Remove the handle, trim plate, and any cover pieces
Remove the valve access panel or open the wall behind the valve
Inspect and note the valve type and pipe connections
Disconnect the old valve from the water lines
Remove the old valve from the wall cavity
Install the new valve in the same orientation and depth
Secure the new valve to framing or supports
Connect the hot and cold water lines to the new valve
Connect the shower outlet and tub spout line if applicable
Solder, crimp, glue, or tighten fittings as required
Turn the water supply back on and check for leaks
Test the valve for proper hot, cold, and mixed water flow
Install the trim, handle, and cover plate
Close the wall opening and repair surrounding surfaces if needed