Begin by turning off the water to the entire house. Locate your main water shut-off valve, which is often in a basement, crawl space, or near the water meter, and turn it clockwise until it stops. Open the shower faucet and the lowest faucet in your house (e.g., a basement sink) to completely drain the water from the supply lines. Place a drop cloth in the bottom of the tub or shower to protect the surface from scratches and falling debris.

Remove the faucet handles, typically held by a set screw, and the surrounding trim plates (escutcheons). Unscrew the shower arm and head. Since we're assuming no rear access panel, you must create an opening from the front. Use a utility knife to score the grout around the tiles you need to remove. An oscillating multi-tool with a grout removal blade is ideal for this. Carefully cut an opening in the tile and backer board large enough to access the valve and surrounding pipes, usually around 12x12 inches centered on the valve body. Always wear safety glasses and a dust mask during cutting.

With the valve body exposed, identify the hot supply, cold supply, and the shower riser pipe. Using a tubing cutter, make clean, straight cuts on all three pipes, leaving several inches of pipe extending from the wall connections to work with. Be prepared for a small amount of residual water to spill out. Once the pipes are cut, the old valve assembly can be pulled out of the wall cavity.

Secure a 2x4 wood block horizontally between the wall studs to serve as a sturdy mounting point for the new valve. Position the new single-handle valve body on the block, ensuring it's set at the correct depth according to the manufacturer's instructions (a plastic guard is usually included for this). Prepare the existing copper pipes and new fittings by cleaning the ends with a pipe cleaning brush and emery cloth. Apply a thin layer of soldering flux to all cleaned surfaces. Cut and dry-fit new sections of 1/2-inch copper pipe and fittings (like 90-degree elbows) to connect the hot, cold, and shower riser pipes to the new valve. Once the fit is perfect, remove the valve's plastic cartridge to protect it from heat, and solder all joints using a propane torch and lead-free solder.

Once the soldered joints have completely cooled, reinstall the valve cartridge. Make sure the valve is in the 'off' position. Go to the main shut-off valve and turn the water back on slowly. Return to the shower and meticulously inspect every new pipe connection and solder joint for any signs of dripping or weeping water. Use a flashlight for a clear view. If a leak is found, you must shut off the water, drain the system, and re-solder the faulty joint. Once you're confident there are no leaks, turn the valve on to test both hot and cold water flow and the diverter function.

With the plumbing confirmed to be leak-free, you can now repair the wall. Cut a piece of cement backer board to perfectly fit the access hole you created. Secure the patch to the wood studs or blocking using corrosion-resistant backer board screws. Apply fiberglass mesh tape to all the seams between the patch and the existing wall. Use a trowel to apply a layer of thin-set mortar over the tape and the entire patch, feathering the edges to make it flush with the surrounding backer board. Allow the mortar to cure fully as per the product's directions before proceeding to tile.

After the wall is repaired and retiled (or if you are using a remodel plate over the existing wall), install the final trim. Slide the large escutcheon or remodel plate over the valve, pressing it flush against the wall. Install the handle onto the valve stem, tightening the set screw with an Allen wrench. Reinstall the shower arm and showerhead using new Teflon tape on the threads. To create a watertight seal, apply a neat bead of 100% silicone caulk around the entire perimeter of the escutcheon plate where it meets the wall. Smooth the bead with a wet finger for a clean finish.