Assuming you know how to sweat pipe joints: Turn off the water. Drain the pipe (otherwise all the energy goes into heating water rather than metal). Heat with a torch and disassemble; don't set the house on fire. Clean THOROUGHLY -- you may want to get a new elbow and do the full wire-brush-and-emery-paper treatment on it, rather than trying to reuse the existing elbow; clean surfaces are essential. Reassemble and sweat-solder, being sure the pipes are heated sufficiently and that you use enough (leadless plumbing) solder. Let cool, turn the water back on, and check for leaks. Repeat if necessary.Or you could attack the thing with epoxy putty... but then you'd have to cut it out and splice pipes next time you have to work on it.