is it an already compiled SWF? if so you need a SWF decompiler, of whcih there are a few, but all of them require $$$. My favorite is SWFDEcompiler by sothink. Anyways what you will end up with is a FLA whcih you will then need Flash to edit that link anyways.
the bad part is this wont always work. Even if you do decompile the swf if they used any wierd components or fancy code you mgiht be stuck not able to recompile properly. On top of that finding the exact place where that link is stored can be a $#@$ to figure out.
So best bet is to just ask the guy that created the swf for his fla file.