Begin by protecting the work area. Lay a drop cloth or old towels in the bottom of the tub or shower stall to prevent scratches from dropped tools. The most critical action is to shut off the water to the entire house using the main water shut-off valve, typically located in a basement, crawlspace, or utility closet. Once off, open the faucet you are replacing and another faucet at a lower level (like a basement sink) to completely drain the water from the system. Gather all necessary tools and the new faucet kit so they are readily available.

From inside the shower, remove the visible parts of the old faucet. Start by prying off the decorative caps on the handles with a small flathead screwdriver or utility knife. This will expose a set screw, which can be removed with an Allen wrench or Phillips screwdriver. Pull the handles straight off. Next, unscrew the escutcheons (the chrome flange plates behind the handles). They may be threaded on or held by screws. If they are stuck, you may need to carefully cut the old caulk seal with a utility knife. Finally, unscrew the tub spout. If it doesn't unscrew, it may have a set screw on the underside that needs to be loosened.

Since we've assumed there is no pre-existing access panel, one must be created. Locate the wall directly behind the shower faucet. Use a stud finder to locate the studs on either side of the plumbing. Plan to cut an opening large enough to comfortably work in, typically about 12x12 inches. Mark your cut lines on the drywall using a pencil and a level. Use a drywall saw to carefully cut along the lines. Be cautious not to cut too deep and damage any pipes or wires that may be inside the wall. Once the square is cut, remove the piece of drywall to expose the faucet valve and plumbing.

With access to the valve, you can now remove it. The valve body is likely soldered to three copper pipes: hot inlet, cold inlet, and the shower riser outlet. Use a tubing cutter to make clean cuts on all three pipes, leaving a few inches of pipe extending from the valve. Have a small bucket and rags ready to catch any residual water. Once the pipes are cut, the valve may also be screwed to a wooden support beam (a stringer) between the studs. Unscrew the valve from this support and remove it from the wall.

Prepare the new valve for installation. Following the manufacturer's instructions, set the valve's depth relative to the finished wall (the tile). This is a critical measurement. Secure the new valve to the wood stringer, ensuring it is level. Now, prepare the pipes for soldering. Thoroughly clean the outside of the existing pipe ends and the inside of the new copper slip couplings using emery cloth and a wire fitting brush until they are shiny. Apply a thin, even layer of flux to all surfaces to be soldered. Fit the couplings onto the existing pipes and then fit short new pieces of pipe to bridge the gap to the new valve. Heat the joint with a propane torch until the flux sizzles, then touch the lead-free solder to the joint. Capillary action will draw the molten solder into the joint, creating a permanent seal. Repeat for all three connections (hot, cold, shower riser).

This is a critical step that must not be rushed. Ensure the new valve is in the 'off' position. Go to the main water shut-off valve and turn it on SLOWLY. Listen for the sound of rushing water filling the pipes. Once the sound stops, return to the access panel. With a bright flashlight, carefully inspect every single soldered joint for any signs of moisture. Look for even the smallest bead of water or a slow drip. Check the connections for a full five to ten minutes to ensure they are completely watertight. If you find a leak, you must shut off the water, drain the system, and re-solder the faulty joint.

With the plumbing confirmed to be leak-free, you can install the new faucet trim from the shower side. Re-install the cartridges or stops into the valve body if you removed them for soldering. Slide the new escutcheons over the valve stems and secure them to the wall. Apply a bead of silicone caulk around the top and sides of the escutcheons to prevent water from getting behind them, but leave the bottom open to allow any trapped moisture to escape. Push the new handles onto the valve stems and tighten the set screws. Finally, apply plumber's putty to the base of the new tub spout or use Teflon tape on its threads, and screw it into place until it is snug and flush against the wall.

The final step is to close the access hole. The highly recommended method is to install a manufactured access panel, which can be purchased at any home improvement store. They are easy to install and provide simple access for any future plumbing work. Alternatively, you can patch the drywall by re-inserting the piece you cut out, securing it with drywall screws to new wood backing, taping the seams with joint tape, applying several coats of joint compound, sanding smooth, priming, and painting to match the existing wall. Once the wall is repaired, clean up all tools, dust, and debris from the work area.