The device the sprayes water from the shower head has broken. The shower head does not unscrew for the shower arm. The shower arm is attached behind the tiled wall. How do I remove the shower arm and attach a new arm and shower head ?

I would make an access panel on the back side of the shower, this way if I ever need to access the shower lines/fixture in the future I can do so, Hope this helped Tim, here is a website that may help:

http://www.ehow.com/how_6071986_install-price_pfister-shower.html

The shower arm should be screwed into an elbow inside the wall. Unscrew the shower arm by putting your pipe wrench on the arm and unscrewing in a counter clockwise direction ( lefty loosey, righty tighty ) . Measure the length of the arm going inside the wall and make sure your replacement has enough length to reach. Teflon tape the inside end ( the end going in the wall ) and wrap a rag around the new arm, then screw into the elbow going clock wise , stopping when it starts getting tight and with the bend facing downward. Install shower head.