Unfortunately, Mobi is not an open or even documented format, so everyone's basically been reverse-engineering it and there's no real way to edit Mobi files as-is.
But you can use various unpacking tools, like the
Ebook-Tools perl module, or
MobiPerl, or
mobiunpack; the latter two of which are the projects of fellow MobileRead members.
These have options to output the specially marked-up Mobipocket variant of HTML that usually gets generated from the original source files, and you can edit and repack once you're satisfied.