Shower spouts and diverters are attached to the wall in one of two ways: they are either threaded onto a galvanized supply pipe, or they are twisted onto a copper supply pipe and held into place with a set screw. Before buying a new shower spout, take a look under the existing spout and look for a small screw underneath. If you find a screw, you have to get that same type of diverter as a replacement. The same goes with the threaded style. Do a little research, buy the correct replacement part, and you will have the job done in no time.
Check the existing diverter to see what type of diverter you have to buy.
Remove the existing diverter held into place with a set screw, use a hex key to loosen the set screw that is holding the existing diverter onto the supply pipe. Twist the spout counterclockwise to remove it from the supply pipe. If the spout will not come loose, place a cloth rag over the spout and use a pipe wrench to break the spout free. If the diverter in your shower is not held ino place in this manner, go to Step 3.
Remove the existing diverter that is threaded onto the supply pipe. Test how tight the spout is threaded onto the supply pipe by attempting to remove it by hand turning the spout counterclockwise. If the spout is too tight to unthread by hand, place a cloth rag over the spout and use a pipe wrench to loosen the spout.
Install the new shower diverter by either twisting the new diverter onto the copper supply pipe and tightening the set screw, or by applying teflon tape to the threaded end of the supply pipe and thread the new diverter onto the supply pipe until it is snug against the wall and in the right position, or close to the wall and in the right position. Squeeze a small bead of silicone caulk around the wall end of the spout to prevent water from collecting behind the supply pipe.