Shut off the water valve on the main water line to stop water flow into your tub. This valve is near the water heater in most homes. Close by turning the valve completely clockwise.
Take off the faucet handle by unscrewing the set screw at the base of the handle. On round faucet handles, take off the plastic cover on top to access the screw. Use an Allen wrench for hex-head screws, and a Phillips screwdriver for Phillips-head screws.
Slide off the handle by gently pulling it backward. Unscrew the rounded dome cap or bonnet, both of which are located under the faucet handle. Rotate them counterclockwise or remove the retaining clip ring around the cartridge, using a pair of pliers. If your faucet has a retaining ring under the bonnet, unscrew it with your pliers and take it off.
Extract the cartridge with your hands by sliding it backward out of the faucet body. Insert a new cartridge into the faucet to replace the old one, or clean out the old cartridge in clean water and replace it. A new cartridge is recommended as it is a faster fix.
Assemble the faucet by replacing the retaining ring, bonnet and dome cap as applicable to your faucet. Replace the clip, if needed. Attach the handle and insert the set screw with the appropriate tool.