I assume you're talking about a frost free valve that is connected to your hose water. These have a 12" long tube with the turning-off parts located on the far inside end so the tube is empty when turned off. The inside end is threaded 2 ways. 1-interior 1/2" threads female and 2- exterior 3/4" male. To replace it, you have to turn the water off (duh) and then with 2 wrenches, hold the supply line while unscrewing the valve. It should slide out through the hole in the sill plate. Replace the opposite way, but it will be helpful to have an assistant outside to help orient the faucet. It must be straight up and down or it won't drain. Note: in freezing weather, do not leave a hose attached.
If you have an outside pipeline, then just turn off the water and take the old faucet off.