Does it leak by dripping out the spigot or from the base of the valve stem? If it's the spigot, you can replace the rubber washer in it to stop the leak. Just undo the packing nut on the top that's around the valve stem and pull the stem out. The washer is screwed to the bottom of the stem.
If it's leaking out of the base of the stem, just tighten the packing nut. This squeezes the packing tighter to the valve stem. The packing allows the valve stem to turn, while sealing the water around the shaft from leaking. Tighten it with the water turned on and the valve shut until it just stops leaking. If you tighten the packing nut too tight, the valve stem will be hard to turn.
If you're intent on replacing the valve, you can remove it with the pipe attached. Then use vise grips around the pipe to remove it from the valve, replace the valve, and then reconnect the pipe. Just keep in mind that you might get another leak where the pipe attaches to the plumbing system if you don't tighten it enough.