You are going to have to find a pipe union that you can open and then start disassembly from there. If there is no union, cut a piece of pipe out, use nipples and install a union. You cannot unscrew the elbow pipe directly because it will tighten on the other end.

What type of material is the elbow? Copper, black iron, etc. Can you post a picture of the leaking elbow? If so, we should be able to tell you the easiest way to go about repairing the leak.

Which parts of the answers are strange kid? If there were no unions installed, then you would have to cut the pipe to replace an elbow (if copper). Of coure if it is indeed black iron or any threaded pipe, then there will be a union somewhere.