Begin by shutting off the water supply to the entire house using the main shutoff valve, typically located in a basement, crawlspace, or near the water meter. Open a faucet on the lowest level of the house (like a basement sink) and the shower faucet itself to drain any remaining water from the pipes. Place a drop cloth on the floor of the shower or tub to protect the surface from scratches and debris. Gather all necessary tools and materials for the project and stage them nearby.

Carefully remove the shower handle, which is often secured by a small set screw requiring an Allen wrench. Once the handle is off, unscrew the faceplate, also known as the escutcheon, from the wall. There may also be a sleeve or tube behind the escutcheon that needs to be removed. Keep all screws and small parts together in a safe place.

This is the most critical and delicate step. Measure the escutcheon plate of the NEW valve. Mark a cutting area on the tile that is slightly smaller than the new plate so it will cover the hole. If possible, align your cuts with existing grout lines. Put on your safety glasses, dust mask, and hearing protection. Use an angle grinder equipped with a diamond blade to slowly and carefully cut along your marked lines. Once the tile is cut, use a hammer and a small chisel to gently tap out the cut tile piece(s). Then, cut and remove the backer board behind the tile to fully expose the old valve and surrounding pipes.

With the valve exposed, identify the hot inlet, cold inlet, shower riser (to showerhead), and tub spout outlet pipes. Take a photo for reference. Using a compact tubing cutter, cut each of the copper pipes connected to the old valve. Make your cuts in a straight, accessible section, leaving as much pipe as possible to work with for the new installation. Once all pipes are cut, unscrew or pry the old valve body from the wood stringer it's mounted to inside the wall and remove it.

Prepare the cut ends of the existing copper pipes. Use a deburring tool to smooth the inside edge of each pipe, then thoroughly clean the outside of the pipes (about 1 inch from the end) with sand cloth or a fitting brush until they are bright and shiny. Do the same for the inside of the new copper fittings. Temporarily assemble the new valve with any necessary couplings and pipes (this is called 'dry-fitting'). Use the manufacturer's included plaster guard or template to position the valve at the correct depth relative to the front surface of the tile. Secure the valve to the wood stringer in the wall with screws.

Remove the plastic cartridge from the new valve body to protect it from heat damage. Place a flame shield behind the work area to protect the wood studs. Apply a thin, even layer of flux to the cleaned ends of the pipes and the inside of the fittings. Assemble the joints. Using a propane torch, heat the fitting (not the pipe) evenly. Touch the lead-free solder to the joint on the side opposite the flame. When the joint is hot enough, the solder will be drawn into the gap, creating a seal. Repeat for all connections. Allow the joints to cool completely without being disturbed.

Re-install the valve cartridge you removed before soldering. Ensure the hot/cold orientation is correct. With the shower valve in the 'off' position, go to the main shutoff and turn the water back on SLOWLY. Return to the shower and use a flashlight to meticulously inspect every new solder joint for any signs of dripping or weeping. If all joints are dry, turn the shower valve on to test its function and to pressurize the outlet connections. Check again for leaks. If any leak is found, you must shut off the water, drain the system, and re-solder the faulty joint.

Once you are certain there are no leaks, you can repair the wall. Cut a piece of cement backer board to fit the opening you created. Secure this patch to the wood studs using corrosion-resistant cement board screws. To ensure the shower remains waterproof, apply a liberal coat of a liquid waterproofing membrane over the entire patch, extending a few inches onto the surrounding existing backer board. Pay special attention to covering all seams and screw heads.

After the waterproofing membrane has fully cured (check manufacturer's instructions), mix a small batch of thin-set mortar to a peanut butter-like consistency. Apply the mortar to the backer board patch using the flat side of a notched trowel, then comb it with the notched side to create even ridges. Firmly press your replacement tile(s) into the mortar with a slight twisting motion. Use tile spacers to maintain consistent grout lines that match the rest of the wall. Allow the mortar to cure for at least 24 hours before proceeding.

Remove the tile spacers. Mix your grout according to the package directions. Apply the grout diagonally across the tile joints using a grout float, pressing it firmly into the gaps. After about 15-30 minutes, use a damp grout sponge to wipe the excess grout off the tile surface. Let the grout cure for the manufacturer-recommended time (typically 24-72 hours). Once cured, apply a grout sealer to the new grout lines to protect them from moisture and staining. Finally, apply a bead of 100% silicone caulk around the perimeter of the new escutcheon plate where it will meet the tile.

With the wall fully repaired, you can install the new shower trim. Slide the escutcheon plate over the valve and secure it to the valve body with the provided screws. The bead of silicone caulk you applied should create a nice seal behind it. Install any sleeves or adapters per the manufacturer's instructions. Finally, attach the new handle and tighten its set screw. Turn the water on and perform a final test of the valve, checking for proper hot and cold operation and a smooth feel.