If you are sweating a pipe that is already in place against a joist, you might have to put something in front of the joist it to keep it from burning. I usually keep a fire extinguisher handy if I am working in this situation.
You can't sweat a fitting on a pipe that is full of water. The fitting will not get hot enough and if you just heat the solder the joint will leak.. If you are trying to sweat a fitting on a vertical pipe that is full of water, use your finger to force a couple of inches of water out of the pipe. You might have to heat the fitting for awhile. This is the worse-case scenario and can be trying. If the pipe is a horizontal one that had water in it, try to bend it slightly to allow the rest of the water to run out of it. Next force a rag into it to dry it out.
You can remove a soldering fitting by simply heating the joint. Use a pair of pliers to work the fitting loose as the solder gets hot. If you are going to sweat a new fitting on, sand as much of the old solder off as possible. If you don't, the fitting might not fit.