Welcome to MobileRead.
It sounds like you need to get hold of
calibre, the free, open source, cross-platform ebook organiser and format converter.
Although if your ePub ebooks have copy protection (DRM) you'll also need to remove that before calibre can convert them.
Explicit advice on how to remove DRM from eBooks isn't permitted here, but I can tell you that Apprentice Alf might be able to help. He has a blog somewhere.