First, you must completely stop the flow of water to your home's plumbing system. Locate the main water shutoff valve, which is typically found in a basement, crawl space, or garage near where the water service line enters the house. It will have a wheel handle or a lever handle. Turn the handle clockwise until it is fully closed. To drain the remaining water from the pipes, open a faucet on the highest floor of your house (e.g., a second-floor bathroom) and another one at the lowest point (e.g., a basement utility sink). This allows air into the system and lets the water drain out, minimizing spillage when you remove the old valve. Place a bucket and towels under the sink where you will be working.

With a bucket in place, the next step is to disconnect the faucet supply line from the angle stop. This is the flexible or rigid tube that runs from the valve to the faucet's threaded tailpiece. Using an adjustable wrench, grip the nut connecting the supply line to the top outlet of the angle stop. Turn the nut counter-clockwise to loosen it. Be prepared for a small amount of water to drip out. Once fully loosened, pull the supply line away from the valve.

This step requires two wrenches to prevent damaging the pipe. Place one adjustable wrench or a pair of channel-lock pliers on the body of the angle stop to hold it steady. This counter-acts the twisting force. Place a second adjustable wrench on the large compression nut located where the valve meets the copper pipe coming out of the wall (the stub-out). Turn this nut counter-clockwise. It may be tight from years of compression and corrosion. Once you break it loose, you should be able to unscrew it the rest of the way. Pull the old valve straight off the pipe.

After removing the valve, the old compression nut and the small brass ring, called a ferrule, will remain on the pipe. Both must be removed. Slide the old nut off. The ferrule is often compressed tightly onto the pipe. The best tool for this is a dedicated ferrule puller, which safely pulls it off without damaging the pipe. If you don't have one, you can very carefully cut a slot into the ferrule with a mini hacksaw (being careful not to cut into the copper pipe itself) and then pry it open with a flathead screwdriver. Once the ferrule is off, use a piece of emery cloth or fine-grit sandpaper to thoroughly clean the end of the copper pipe. Remove all mineral deposits, old putty, and oxidation until the copper is shiny and smooth. This ensures the new ferrule will create a perfect, watertight seal.

Take the new angle stop and disassemble it. Slide the new compression nut onto the cleaned pipe first, ensuring the threads face towards the end of the pipe. Next, slide the new brass ferrule onto the pipe, with its tapered end pointing towards the end of the pipe (where the valve will sit). Push the body of the new angle stop firmly over the pipe until it seats snugly against the ferrule. Slide the compression nut forward and hand-tighten it onto the valve's threads. Now, use the two-wrench technique again: one wrench to hold the valve body stationary, and the second to tighten the compression nut. Tighten the nut about one-half to three-quarters of a full rotation past the point where it was hand-tight. Ensure the valve's handle is in the 'off' position (handle perpendicular to the outlet for a quarter-turn valve).

Reconnect the faucet supply line to the threaded outlet on the new angle stop. Hand-tighten the nut, then give it a gentle snug with an adjustable wrench (usually about a quarter-turn). Make sure all other faucets in the house that you opened earlier are now closed. Go to the main water shutoff valve and turn it on SLOWLY. Opening it slowly prevents a sudden surge of pressure (water hammer) that could damage pipes. Return immediately to the sink and closely inspect the new valve. Look and feel for any water dripping from either the large compression nut at the wall or the smaller supply line nut. Wipe the entire valve and connections with a dry paper towel to make any moisture instantly visible. If there are no leaks, slowly turn the handle on the new angle stop to open it and restore water to the faucet. Check for leaks one more time with the water flowing.