Flash isn't too easy to modify to be honest. It depends on what you want to do - like already said changing the music, sound clicks, fade times for slideshows, etc. is easyish, but creating new pages or really modifying anything past these simple things can get pretty hard and confusing. If you know html that will help, but actionscript doesn't really resemble html. Do you have any c++ or java programming (even a little) experience? I found my c++/java experience very helpful in learning actionscript.
If you want to either post or PM me with a link to the template and what you want to do with it I might be able to help you. I'm learning flash right now and trying to build my website using it. Totally feel free to shoot me a PM if you have any questions.
Good luck.