This is outside my normal scope of blog posts but a friend on facebook was struggling with this so I decided to put this on here since he's having a busy day and I haven't slept since 4pm yesterday.
This will only work with non-DRM (Digital Rights Management) protected files.
The file may have a different extension but I tested it with a different file extension and simply changed the file extension to .mobi from whatever it was.
Things you'll need:
1. mobi2html win32 tool
3. Mobipocket creator or another tool to repack the .mobi This one creates a .prc file that seems to open fine in Kindle reader for PC
Unpack the mobi2html tool and run it. (I've been told there is a false positive on some antivirus programs out their but according to multiple sources it truly is a FALSE positive and can be ignored.)
It will ask you for the location of the .mobi file you're trying to "explode" / unpack. Locate the file and continue.
This little tool extracts the contents into a simple HTML file that you can then edit simply with Notepad++.
When you are done editing the file save it and open Mobipocket creator and choose the files accordingly to repack into the new .mobi/.prc file that you'll be creating. Don't forget any images from the cover or table of contents it may have unpacked.
Click Build and off you go. Once it's done you'll have a new .prc file you can then open in your reader.
Again I don't know that this works on DRM protected files. You're on your own if you're trying to do that. Also if you DEFINITELY need it to be ".mobi" you can try simply changing the file extension but I didn't try to as I didn't actually have an actual ".mobi" file to start with.
Anyhoo, good luck and God Bless.