Disconnect the water line where it connects to the valve using a crescent wrench. Next, remove the angle stop -- use two wrenches, one to hold the compression nut and one to hold the angle stop. To loosen, remember to turn the compression nut, not the angle stop.
To reattach the new supply line and angle valve, first remove the compression nut and slide it over the copper nipple. Replace the compression sleeve and tighten using two crescent wrenches -- one wrench holds the angle stop in place, the second wrench tightens the nut.
The stem is the valve that's controlled by the faucet handle. To clean it, first remove the stem, again using two crescent wrenches, one to hold the nut underneath the sink and one to turn the top. Once removed, disconnect the part of the stem for thorough cleaning.