I'm assuming since your file has an
M4B extension, you're working with an audiobook. You should be able to use Audacity to open the .m4b file and export it as an MP3 file. The m4b extension is just a fancy name for an AAC file.
I would recommend the 1.3 beta version - even though it's labeled as a beta, it's very stable. You'll also need to install the LAME MP3 encoder (to export the mp3 file) and the FFmpeg Import/Export library (to read the .m4b file).
The first time after you open Audacity, it should automatically find the LAME and FFmpeg installs. If not, you can have it search for them by going to
Edit -Preferences (or press Ctrl+P), go to "Import / Export", and click the "Find library" buttons if nothing is listed (see here for detailed instructions)
Having never converted a file that large, I'm not sure how long it will take. Once Audacity is installed, open the .m4b file, and then choose
File -> Export... and save it as an MP3 file. Audacity will give you the chance to fill out the ID3 information, so you can have the proper author, etc, listed for the MP3 file.