You can convert almost any format into any other format.
You can do this on the Web at http://convert.com and you can download the “calibre” software free from http://calibre-ebook.com.
If the book is protected by DRM, you may not be able to convert it.
A search engine will let you find sites that claim to have software that will strip DRM from e-books and allow you to convert them.
Their claims may or may not be true. If they are true, using the software may or may not be legal.