Shower mixer valves usually have hot and cold copper lines or pipes going into the bottom of the valve, and one copper line coming from the top of the valve up to the brass shower head fitting. The valve itself usually is attached to a piece of horizontal wall stud, either with screws or metal strapping, which in turn is screwed to the vertical wall studs on both sides. All three copper lines will have to be cut through and the valve removed from the wall stud.
Turn off the water to the mixer valve at the house's main shutoff valve. Mark the two copper lines going into the bottom of the valve, and the copper line coming out of the top of the valve with a felt tip pen. Mark them all at least 6 inches away from the valve. Place a tubing cutter over one of the pipes and tighten the blade over the mark. Rotate the cutter once. Tighten the cutter again and rotate a second time. Continue in like fashion until the pipe is cut through. Cut through the other two pipes following the same method.
Remove the screws or metal strapping holding the mixer valve to the horizontal stud and remove the valve. Attach the new valve to the stud with either screws or strapping. Sand the cut ends of the two existing pipes below the valve, and the one cut pipe end above the valve using emery cloth. Sand the two inlets on the bottom of the valve and the one outlet on the top of the valve, also using emery cloth. The inlets and outlet will have copper pipe installed into them and soldered into place.
Measure the distance between the outlet on the top of the valve and the end of the cut pipe above it. Make sure that you allow for the pipe being pushed into the outlet when you calculate the length of pipe needed, usually about 1/2-inch, and add this to the required length of the pipe. Cut a new piece of copper piping to that length using the pipe cutter. Follow the same instructions for the two copper pipes coming into the bottom of the valve.
Sand the ends of all three new pieces of pipe with emery cloth, as well as the insides of three straight couplings. Using a small brush, apply a thin layer of soldering paste (flux) to all sanded areas, that is all pipe ends, insides of couplings and insides of the valve inlets/outlet.
Push the couplings onto the three cut ends of the existing pipes, one above and two below the valve. Push the new cut piece of pipe into the coupling above the valve and the other end into the outlet on the top of the valve. Push the new pieces of pipe into the two couplings below the valve and the other ends into the inlets on the bottom of the valve.
Uncoil 12 inches of solder from its spool, and bend the last two inches 90 degrees. Turn on the propane torch. You will now solder all three couplings, as well as the one inlet and two outlets. They will all be soldered in the same way. Start at the coupling above the valve, then work your way down to the outlet and two inlets, and finally the two couplings below the valve.
Heat the coupling by moving the flame from side to side. Do the same at the back of the coupling. When you hear the flux sizzle, take the flame away. Touch the seam between the pipe and coupling with the end tip of the bent 2 inches of solder. If it melts, apply 3/4-inch of solder to the seam. Capillary action will suck most of the solder down between the coupling and pipe, creating a seal. Complete soldering the coupling before wiping away any excess solder with a rag. Be careful, as the coupling will still be hot. Follow the same steps for the inlets/outlet and two remaining couplings.
Turn the water supply back on after all couplings and inlets/outlet have cooled down.
Determine the diameter of new pipe and couplings needed by checking the existing copper pipes.
Be aware of all flammable objects when you solder. Keep a water spray bottle close at hand.