Replace a shower arm by wrapping a cloth around the existing arm and unscrewing it counterclockwise with an adjustable wrench. Clean old plumber tape and debris from the wall fitting threads. Apply 3 to 4 wraps of fresh Teflon tape clockwise on the new arm threads. Hand-tighten the new arm into the wall fitting then snug with a wrench. Avoid overtightening which can damage the fitting inside the wall. Attach your shower head and test for leaks.