Many showers are controlled by a cartridge that turns with the handle like a screw. The turning of the cartridge controls the flow of water from the showerhead. When the end of the shower cartridge that holds the handle in place becomes worn or its parts begin to decay, you will need to repair the shower cartridge by replacing it. Replacement cartridges are available from the manufacturer of the shower at home improvement and hardware stores. If you do not know the model of your shower cartridge, inspect the side of it for a model number and manufacturer.
Turn off the water to the bathroom by turning your house's main water-supply valve to the off position.
Pry off the cover of the shower handle using a screwdriver. Do not to break the cover, as you will need to replace it later.
Lift up the retaining clip that holds the shower cartridge in place by inserting the end of your screwdriver into the hole on the clip and lifting it up and out of the cartridge.
Place the cartridge-removal tool that comes with your replacement cartridge over the cartridge, then twist it back and forth using a wrench. This loosens the cartridge. Pull on the cartridge and remove it from the shower.
If the shower cartridge will not come out, place a cartridge puller over the cartridge and turn the screw on the end to tighten it. Pull on the puller to remove the cartridge. A cartridge puller is a special tool available from most hardware stores.
Insert the new cartridge, aligning it properly in the hole so that it slides in. Replace the handle.