First make sure the tub is dry (so you don't slip) and the shower taps are turned off no need to cut off the water supply to the entire house.
Step 1
Cover the jaws of your adjustable pliers or wrench with duct tape
so you don’t damage the old showerhead’s finish (in case you want to use it later); the tape will also help the grip. Hold the shower arm firmly with one hand and slowly unscrew the showerhead, turning counterclockwise.