$ gem install swf_ruby

or

$ git clone git://github.com/tmtysk/swf_ruby.git
$ cd swf_ruby
$ rake install

$ swf_dump samples/sample.swf
SetBackgroundColor, offset: 20, length: 5
DefineFont2, offset: 25, length: 34
DefineEditText, offset: 59, length: 50
PlaceObject2, offset: 109, length: 11
DefineBitsLossless2, offset: 120, length: 178
DefineShape, offset: 298, length: 55
:
DefineBitsJPEG2, offset: 623, length: 10986
:

$ swf_jpeg_replace samples/sample.swf 623 samples/bg.jpg > samples/sample2.swf
# <623> is offset to DefineBitsJPEG2 resource getting by 'swf_dump'.

$ swf_lossless_replace samples/sample.swf 120 samples/icon.gif > samples/sample3.swf
# <120> is offset to DefineBitsLossless2 resource getting by 'swf_dump'.

$ swf_as_var_replace foo.swf bar piyo > foo2.swf
# <bar> is variable name. <piyo> is new value to the variable.

$ swf_sprite_replace foo.swf bar piyo.swf > foo2.swf
# <bar> is instance variable name of movieclip. <piyo.swf> is new movieclip file to replace.