The first critical action is to completely stop the flow of water to your entire house. Locate your main water shutoff valve, which is typically found in the basement, a crawl space, a utility closet, or outside near the water meter. Turn the valve handle clockwise until it is fully closed. For a lever-style valve, turn it so the handle is perpendicular to the pipe. Once closed, relieve the pressure in the pipes by opening a faucet on the lowest level of your home (like a basement sink or outdoor spigot) and the faucet connected to the valve you are replacing. Let them run until the water stops flowing.

Prepare your work area. Clear everything out from under the sink to give yourself ample room to work. Place a bucket directly under the angle stop you are replacing to catch residual water that will drain from the pipes. Lay down several old towels around the bucket to protect the cabinet from water damage.

Use an adjustable wrench or basin wrench to disconnect the faucet supply line from the outlet of the old angle stop. Turn the coupling nut counter-clockwise to loosen. Be prepared for a small amount of water to drip out. Tuck the disconnected supply line out of the way.

This is the most critical step. You must use two wrenches. Place one wrench on the body of the angle stop and turn counter-clockwise. At the same time, use a second wrench to grip the copper pipe coming from the wall (the stub-out) to hold it firmly in place, preventing it from turning. Loosen the large compression nut that secures the valve to the pipe. Once loose, you can finish unscrewing it by hand and slide the old valve off the pipe.

With the valve removed, you will see the old compression nut and a brass ring called a ferrule on the copper pipe. Both must be removed. The nut should slide off. The ferrule can be stubborn. The best method is to use a dedicated ferrule puller tool, which grips behind the ferrule and pulls it off without damaging the pipe. Alternatively, you can carefully cut a slot into the ferrule with a small hacksaw or rotary tool, being extremely careful not to cut into the copper pipe itself. Once cut, you can pry it open with a flathead screwdriver and remove it.

Once the old fittings are off, thoroughly clean the end of the copper pipe stub-out. Use a piece of emery cloth or fine-grit sandpaper to polish the outside of the pipe where the new ferrule will sit. The goal is to remove any old pipe dope, corrosion, or burrs, creating a perfectly smooth surface for a watertight seal. Wipe the pipe clean with a dry rag when finished.

Take your new angle stop and disassemble the compression fitting. 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. Push the body of the new angle stop onto the pipe until it seats firmly against the ferrule. The pipe should go about 1/2 inch into the valve body. Hand-tighten the compression nut onto the valve body until it is snug.

Using the same two-wrench technique as before, tighten all connections. Use one wrench to hold the body of the new angle stop steady, preventing it from turning. Use the second wrench to tighten the compression nut onto the valve. After hand-tight, give it one-half to one full turn with the wrench. Do not overtighten, as this can damage the ferrule or the valve. Next, reconnect the faucet supply line to the outlet of the new valve. Hand-tighten the coupling nut, then give it a gentle quarter-turn with a wrench.

Double-check that the new angle stop's handle is in the closed (off) position. Go back to your main water shutoff and turn it on SLOWLY. Let the pipes fill with water gradually to prevent water hammer. Listen for any sounds of rushing water. Go back to the sink and meticulously inspect the new valve for any signs of water, no matter how small. Check around the large compression nut and the faucet supply line nut. If everything is dry, place a dry paper towel under the valve, then slowly turn the handle on the new angle stop to open it. Let the water run at the faucet for a minute to clear out any air. Perform one final check for leaks at all connections.

Once you are confident there are no leaks, dry the entire area, including the pipes, valve, and cabinet floor. Remove the bucket and wet towels. Organize your tools and return any unused supplies. It's a good practice to check on the new valve again after an hour and then a day later to ensure no slow leaks have developed.