"What are the advantages and dis-advantages of using open source software?"
If you are intested, a good start would be the classic book on Open Source development: Eric Raymond's "The Cathedral and the Bazaar". He discusses two software development styles, and in particular talks about the development of fetchmail (a mail retrieval and forwarding utility), Raymond's Open Source project.
There is more to it than that, of course. You can read it here: click here
"How do you edit open source software?"
You have the source code. I would start by opening it in a pager or an editor and looking at it...