Before beginning any work, you must shut off the water to the faucet. Look beneath the sink for two small valves, known as shutoff valves or stops, connected to the wall. One supplies hot water and the other cold. Turn the handles of both valves fully clockwise until they stop. Once closed, turn both faucet handles to the 'on' position on the sink. A small amount of water will drain out, and the flow should stop completely. This depressurizes the lines and confirms the water is successfully shut off.

Lay a soft towel in the basin of the sink. The primary purpose of the towel is to cover the drain opening, which prevents any small screws, clips, or other faucet parts from accidentally being lost down the drain. As a secondary benefit, the towel also protects the surface of your sink from being scratched by tools.

Begin disassembly by locating the decorative cap on the faucet handle; it might be on the top or front. Carefully pry this cap off using the edge of a small flathead screwdriver. Underneath the cap, you will find a handle screw. Using the appropriate tool, either a Phillips head screwdriver or an Allen wrench, turn the screw counter-clockwise to loosen it. You don't always need to remove it completely. Once the screw is loose, grip the handle firmly and pull it straight up and off the faucet body.

With the handle removed, the top of the cartridge is exposed. You will see a packing nut or retaining bonnet threaded over it. Use channel-lock pliers or an adjustable wrench to grip this nut and turn it counter-clockwise to unthread it completely. Once the nut is removed, grasp the stem of the cartridge with the pliers and pull it straight up out of the faucet body. The cartridge may be snug, so a gentle wiggling motion may be necessary to free it. Pay close attention to its orientation (how it's aligned) before you fully remove it.

After removing the old cartridge, look inside the empty faucet valve body. Use a clean rag, perhaps moistened with a small amount of white vinegar, to thoroughly clean the interior, removing any visible gunk, sediment, or mineral deposits. A clean surface is essential for the new cartridge to seal properly. Take the old cartridge you just removed to your local hardware or plumbing supply store. Show it to an associate to find an exact replacement part. This is the most reliable way to ensure you purchase the correct Replacement Faucet Cartridge for your specific faucet model.

Prepare the new cartridge for installation. If it came with new O-rings, apply a very light coating of plumber's grease to them. This helps them seat without tearing and creates a better long-term seal. Align the new cartridge based on the orientation you noted from the old one (there are often tabs or notches that fit into corresponding slots in the faucet body). Carefully slide the new cartridge down into the faucet body until it is fully and firmly seated.

Follow your disassembly steps in reverse. First, thread the packing nut back over the new cartridge and tighten it with your adjustable wrench or channel-lock pliers. Tighten until it is snug, but do not overtighten. Next, slide the faucet handle back onto the cartridge stem, aligning it correctly. Re-insert and tighten the set screw using your Allen wrench or screwdriver. Finally, press the decorative cap firmly back into its place on the handle.

Return to the shutoff valves under the sink. Turn their handles slowly counter-clockwise to restore the water supply. Opening them slowly prevents a sudden 'water hammer' surge. With the water on, turn the repaired faucet handle on and off a few times to bleed any air from the lines. Finally, with the faucet turned off, carefully inspect the spout and the area around the base of the handle you repaired. Watch for several minutes to ensure there are no drips or slow leaks. If it remains dry, the repair is successful.