If they aren't DRMd/secured mobi ebooks, Calibre will do the changeover, BUT... I do this for a living (MAKE ebooks and print books for publishers and self-publishers), and going from ePub to Mobi with Calibre will always give you better results than going the other direction. Not that you have that option, I understand. In fact, my process is to always make the ePub first and move from there to Mobi/PRC to get best results in end files. No offense, but if you want to convert something that is already in it's final Mobi state to something else, you're never going to come up with the kind of results I get making a file from scratch and going through the logical steps from one format to the epub or mobi books. I used Vibosoft mobi to epub converter: this is the guide how to convert mobi file to epub format.
or Coolmsuter app:
http://www.coolmuster.com/epub/mobi-to-epub.html
all of them works,but it is paid!