Remove the Handle and Spout
Before proceeding be certain the water to the faucet is shut off at the valve underneath the sink. With a screwdriver remove the screw attaching the handle to the faucet assembly. Lift and remove the handle.
Next, use adjustable pliers to remove the retaining nut and use needle-nose pliers to remove the small retaining clip. You should now be able to lift and remove the faucet spout and stem sleeve.
Replace the O-Ring
You can roll the o-ring from the base of 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 match for its replacement.
Before installing the new o-ring, coat it with petroleum jelly and then roll it onto the assembly.
Clean or Replace the Cartridge
Using a cartridge puller, remove the cartridge by pulling it straight up. Using pliers to remove the cartridge may damage it.
You can clean an old cartridge with vinegar to remove build up.
Slip the cleaned or replacement cartridge back into the stem and reassemble the faucet. If hot and cold water is reversed, remove the handle and turn the step 180 degrees.