To edit .swf files you need:
a) The original .fla files; or
b) A decompiler that is capable of getting something very close to the
original .fla files
Colleagues have had varying degrees of success decompiling .swf but it's
never a "one way works" type process.
You'll need to look through Google or AltaVista for a decompiler and
find one that works. Your mileage will vary depending on how complex the
.swf is and how well protected against decompiling it actually is.
I don't believe that the YUI tools can help with this problem at the
moment and I don't know of any places that might offer free advice; I
hope this advice helps.