You can decompile, edit, and recompile SWF using swfmill. This tool turns a SWF into XML and back again, e.g.:

swfmill swf2xml movie.swf movie.xml

swfmill xml2swf movie.xml movie.swf

However, whether this will be useful to you depends on how the SWF is put together. You can edit images, videos, audio etc within the movie using this method. They are present in the XML file as base64 encoded blocks which you can replace.

Editing text is much harder because text in SWF files is usually converted to curves so that the fonts do not need to be embedded.