After turning off the water under the sink, remove the old cartridge.
Tip: Check to see whether a solid-brass replacement cartridge is available. Brass cartridges far outlast plastic.
First remove the decorative cap to expose the screw holding the handle in place. Use a Phillips-head screwdriver to remove the screw, then remove the handle and decorative sleeve.
Inspect the old cartridge. If it has more damage than a broken O-ring, you'll need to replace it.
Drop in the new cartridge and push it firmly into place.
Carefully replace the retaining clip and sleeve.
Reattach the handle and secure it with the screw.
Check the flow of water to make sure that the hot- and cold-water adjustments work properly. If hot water flows out when the handle is turned to cold, remove the handle and rotate the cartridge 180 degrees. Reattach the handle and check your results.
If everything works properly, replace the decorative cap and you're done.