Before you replace a shower fixture, start by shutting off the water to the bathroom. Remove the exterior frame and trim of the fixture. Enlarge the hole to remove the old valve. Then, install a new valve, repair the wall, and attach the new fixture.
After shutting off the water to the bathroom, locate and turn on a faucet on a lower level of the house if one exists. This process drains the water from the lines.
Some shower fixtures are caulked to the wall to prevent leaks. In this case, use a razor or knife to break the seal and pull off the frame.
When cutting into the wall, make a hole that is large enough to fit both the old valve and the new valve. The old valve may be connected to the home's plumbing with soldered sections, threaded pipe fittings or both. Cut through the soldered sections, twist off the threaded connectors, and use the reverse process to mount the new valve.
Once the valve is installed, test the fit of the new fixture. If it fits, repair the hole with drywall and cover it with the shower tile before you screw the new fixture in place.