Calibre is a software program used to manage and convert your ebooks to/from various file formats. Here is a link to download calibre. If, after installing it, you are still confused, there is a calibre subforum here on mobileread
https://www.mobileread.com/forums/forumdisplay.php?f=166
Download calibre here
http://calibre-ebook.com/download
It is a fantastic program!!