Flash files are usually .SWF files that were compiled from the source files, .FLA afaik. It is possible to reverse engineer some SFW files back to their original FLA source but I've never found the automatic programs that does this for you to be that good in doing it.
Flash video files on the other hand, from eg. Youtube, are .FLV files that can be converted into AVI files and then edited in an array of different software applications. Legal? ... Meh, who cares. What file are you trying to edit?
SWF it is. You need to decompile that sonofagun. A simple google for 'swf to fla' or 'swf decompile' will give you a slew of results - try them all to see what works the best for you.