Before starting, ensure the work area is safe and you are prepared. Locate the main water shutoff valve for your home (often in the basement, a crawlspace, or near the water meter) and turn it clockwise until it is fully closed. To drain the lines, open the hot and cold handles of the shower faucet you are replacing, as well as a faucet on a lower floor, like a basement sink. Lay a drop cloth in the tub or shower base to protect it from scratches and debris. Put on your safety glasses.

To access the valve, you must create an opening in the wall. Since we assume there is no access panel, you'll need to cut through the tiled shower wall. First, use a stud finder to locate the studs on either side of the existing valve. Using a pencil and a level, mark a rectangular opening that is large enough to comfortably work within (e.g., 12x12 inches). Put on a dust mask. Use an angle grinder equipped with a diamond blade to carefully and slowly cut along your marked lines through the tile and backer board. Remove the cut-out section.

First, from the front, use a screwdriver to remove the three faucet handles and the trim plates (escutcheons). Now, working through the access hole, identify the hot, cold, and shower riser pipes connected to the old valve. Since the pipes are copper, use a compact tubing cutter to make clean cuts on the straight sections of the pipes leading into the old valve body. Leave as much pipe as possible to work with. Once all pipes are cut, the old valve body should be loose. If it is screwed to a support block, remove the screws and pull the old valve out of the wall.

Securely mount the new single-handle valve body inside the wall. Most valves must be attached to a wood support block (a 2x4) running between the studs. Cut a piece of 2x4 to fit snugly between the studs and screw it into place. Position the new valve body so that its center aligns with the hole you cut, and crucially, at the correct depth. The valve instructions will include a plastic guard or guide showing the correct depth relative to the finished wall surface. Secure the valve to the 2x4 support with screws. Now, prepare the pipes. The old separate hot and cold lines that went to the side handles need to be capped off. Cut them back and install push-to-connect caps. You will then run new 1/2-inch copper pipe from the main hot and cold lines to the new valve's inlets. Use your tubing cutter to cut new pieces of copper pipe as needed to connect the existing hot, cold, and shower riser pipes to the new valve. Use a deburring tool on the inside and outside of every cut pipe end to ensure a smooth surface for a watertight seal.

This is a crucial verification step. Ensure the new valve is in the closed/off position. Go back to the main water shutoff and turn it on slowly. Listen for the sound of rushing water filling the pipes. Return to the shower and use a flashlight and a dry paper towel to meticulously inspect every new fitting and connection. Wipe each joint with the paper towel; any moisture will be immediately obvious. If all joints are dry, turn the faucet handle on, directing water to the tub spout first, then use the diverter to send water to the shower head. Cycle between full hot and full cold. Check all joints again for leaks while the water is running. If any leak is detected, turn off the main water supply immediately, release pressure, and remake the leaking connection.

With the plumbing confirmed to be leak-free, it's time to close the wall. Since we are using a remodel plate, the goal is to create a solid, flat surface for it to mount against. You can attach pieces of wood or metal strapping inside the wall around the hole to serve as a backing. Then, cut the piece of tile you removed to fit tightly around the new valve body. Secure it back in place using a strong construction adhesive or epoxy against the backing you installed. This doesn't need to be a perfect, waterproof tile job, as the large remodel plate will cover the entire area and its seams.

Follow the manufacturer's instructions for your specific faucet model to install the final trim pieces. Slide the foam gasket and the large remodel plate over the valve and press it against the wall. Screw it securely in place. Slide the handle sleeve over the valve stem, then attach the handle with the provided set screw, which is often tightened with an Allen wrench. Ensure the handle is oriented correctly (e.g., pointing down for 'off'). Screw on the new tub spout, using a small amount of plumber's putty or Teflon tape on the threads for a good seal. Finally, screw on the new shower head.