The decomplier should rip the .swf to an .fla. Open the .fla and make the edits, then test the movie to create the revised .swf

That site looks like a single flash file so you're probably looking in the library for movie clips generally called page1, page2, etc.

It's a drag the person who edited that template didn't give the site owner the source files. I'd probably center the flash on the page too.