If your faucet is dripping out of the spout. This is how to repair it if you have a faucet with this type of cartridge system
For details see diagram attached.
In the order you see. Valve nut, o-ring (may or may not be used in your valve) Valve Cartridge, Cup Washer and Spring.
Turn off the water to the valve at the water supply under the sink if you have shut-off valves there. Otherwise you will have to turn off the water to the whole house. Open some of the other faucets in the house if you have to turn the whole house water supply off at the water meter valve to reduce the line pressure. Open the handles of the faucet you are working on as well after you turn of the water either way.
If it has two handles there should be a set screw that holds the handle on. You will need the appropriate allen wrench for the set screw. Loosen that and wiggle the handle off the valve stem carefully. There is a brass nut that holds the valve cartridge in the valve body. Using a small pair of channel lock pliers or the appropriate box wrench. Loosen and remove that brass nut. Once that is off, grasp the valve stem with the channel locks and gently wiggle the valve as you pull upward. There will be some suction that wants to hold it in so this may take some finessing to loosen and remove. Be firm but gentile. Make sure you pay attention to the position the cartridge is in so you can place the cartridge back in the same position it was when you removed it. Otherwise, the handle will not be in the proper position when you put it back on.
Once you have the cartridge out. There should be a small black cup washer that sits in a hole on top of a spring at the bottom of the inside of the valve body. You should replace both the spring and cup washer.
You can find them at your local hardware store or big box DIY store.
I use a real small screwdriver to lift the washer and spring out.
When you go to place the new cup washer and spring back in place.
Place the spring in the whole with the larger side down. Then, to place the cup washer back. Place the cup washer on the small screwdriver and hold it with your index finger and lower it to the whole over the spring. Once you have the cup washer over the spring. You have to get the wider bottom part of the washer into the hole. Holding the washer down with your index finger. Use the small screwdriver tip to push the side of the washer into the hole working your way around the washer. Pressing the sides of the washer inward so the bottom edge pops into the hole over the spring. Once you fell the washer pop in and seat. Push straight down to get it fully placed. Then reverse the procedure you used above to remove the cartridge. Paying careful attention to the position the cartridge was in when you removed it and replacing it in the exact same position. There are o-rings or an o-ring around the cartridge so you'll have to wiggle the cartridge and press down to get it to seat level. Place the nut back over the cartridge and tighten down firmly. Replace the handle and tighten the set screw.