Fix a Two Handle Tub Cartridge Faucet Leak
Remove the Handle
Before proceeding be certain the water supply is turned off. Using a small knife or screwdriver, carefully pry of the trim cap off the top of the handle.
With a screwdriver remove the screw that holds the handle to the stem and remove the handle by pulling it. Slide the ring plate off the stem.
Remove the Cartridge
Use pliers or a specialty stem stocket to remove the retaining nut. With pliers, graps the end of the cartridge and pull it out.
Replace the O-ring
You can roll the o-ring from the stem assembly. If it is stuck in place use a screwdriver to get it free. Be careful not to cut the o-ring as you will need to use it to a match for its replacement.
Before installing the new o-ring, coat it with petroleum jelly and then roll it onto the assembly. Reassemble the faucet and test for leaks.