The first step is to turn off the water supply to the shower. This will prevent water from flowing out of the shower head when you remove the valve. To turn off the water supply, find the main water valve for your home and turn it to the "off" position.
Once the water supply is turned off, you can remove the shower handle. To do this, use a Phillips head screwdriver to remove the two screws that hold the handle in place. Once the screws are removed, you can pull the handle straight off of the valve stem.
The next step is to remove the valve cartridge. To do this, use a flathead screwdriver to pry the retaining clip out of the valve body. Once the clip is removed, you can unscrew the valve cartridge by turning it counterclockwise.
Once the valve cartridge is removed, you can inspect it for damage. If the cartridge is damaged, it will need to be replaced.
If the valve cartridge is not damaged, you can reassemble the shower valve. To do this, simply reverse the steps that you took to remove the valve cartridge.
Once the shower valve is reassembled, you can turn on the water supply. To do this, simply turn the main water valve for your home back to the "on" position.
Once the water supply is turned on, you can test the shower valve to make sure that it is working properly. To do this, turn on the shower and check for leaks. If there are no leaks, the shower valve is working properly.