If the shower arm is newer, and is a soft silver colored brass tube, and you are sure you aren't going to use it any more, you can put a 16 or 18 inch pipe wrench on it, and turn it counter clockwise. The pipe wrench will crush the tube, but after it is crushed you can tighten the wrench and keep turning. If you aren't comfortable with that, you have already removed the shower head, you can pull the decorative piece at the wall (its called an estucheon) away and look into the cavity behind it with a flashlight. You'll see threads on your shower arm screwing into either a "Tee" or a ninety degree bend. Where those threads screw together, you are experiencing a rusting together of the pipes. You can go to any good hardware store and buy penetrating oil. Usually the oil will come with a spray areosol head, and also a small 1/8 inch diameter "straw" which you can poke into the aerosol spray head, and get a concentrated spray right exactly onto those tight threads. Let it sit and penetrate for about two hours, and you should be able to unscrew the shower arm. If it still won't budge, spray more penetrating oil on it, set the pipe wrench into the shower arm so that it really gets a good grip, and take a 24 inch piece of steel pipe (or hard plastic), slide it over the pipe wrench handle to fashion a longer handle (called a "cheater") which will give you more leverage, and I bet that poor shower arm will turn. But if it wont turn, make your cheater three feet long then turn it. When you finally get that arm out of there, and you're going to screw the new one in, use some teflon tape wrapped tightly around the threads before you screw it in. All the pressure you are applying to get the old one out, may cause a small bit of stretching of the metal threads, which would be a potential leak inside your wall.
: In my attemt to replace my old shower heads, I've discovered that the shower arms were tightened too
: much. I am having difficulty removing the old arm. It just won't budge. How do I go about removing it?
: Thanks, Sikada