Replacing a tub diverter is an easy job in most cases. There are those exceptions, however.
The tub diverter should have been caulked in place where the rear of the spout meets the shower wall. Cut that caulking free with a sharp knife being careful to avoid cutting the shower wall surface.
Usually, you can score the caulking or lift up an edge and then peel the rest off. If someone has caulked the spout with silicone caulk you may need to exercise patience in removing it. Silicone caulk is very difficult to remove. Use a very sharp knife to cut away at it.
Feel the underneath side of the tub spout. If there is an opening at the rear of the spout where it meets the shower wall, you will need to loosen an allen screw up inside that opening. The best tool to do this with is a balldriver allen wrench. http://www.bondhus.com/products/screwdrivers/body-0.htm. This is an allen wrench that has a ball-like tip. That allows you to tilt the allen wrench at an angle and still loosen the allen set screw.
After fully loosening the set screw, turn the tub spout gently to break the seal and pull it toward you to remove it. Install a new tub diverter spout with the same style connection, i.e, an allen set screw at the rear. Hold the diverter spout firmly against the shower wall or as far as it will go while tightening the setscrew. You can tighten the setscrew about hand tight with the spout turned 90 degrees to one side so you can get to the setscrew easily. Then turn the spout so it faces down into the tub and tighten the setscrew the rest of the way.
If there is no opening at the bottom rear of the tub spout then the tub spout unscrews from a nipple. Place the handle of a pair of pliers or channellocks into the spout opening and turn it counterclockwise. If you are going to throw the tub spout away just grab the end of the spout with a pair of channellock or waterpump pliers or even a pipe wrench and unscrew it. Sometimes corrosion has totally locked the spout in place and you will have to exert quite a bit of force to remove it. In doing so, you might damage the threaded spout nipple. If you have to replace it and can’t get a wrench on it to unscrew it, use a nipple extractor that you can buy at any hardware store or plumbing outlet.
Apply joint compound to the threads of the new nipple and screw it into place using the nipple extractor to tighten it, if necessary. Apply joint compound to the external threads of the spout end of the nipple and install the tub spout. Be sure to caulk it where it meets the shower wall. Trim the acrylic latex caulk with silicone with a wet finger tip or a caulk trim tool. Let the caulk dry at least 24 hours before getting it wet.
A note of caution: The tub spout nipple fitting should have been anchored inside the wall to supporting lumber but not all are. Use extreme caution when removing the old tub diverter spout or tub spout nipple. If you think the fitting in the wall is loose or not supported at all, or suspect the nipple fitting or its connections in the wall have been damaged, you must open up the shower wall to repair or replace it.
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