To convert from iBooks, you will first need to use a Computer to extract the epub files from your iOS Device, there’s a guide here: http://www.igeeksblog.com/how-to-export-ibooks-from-iphone-ipad-to-computer/
Then simply drag the epub into the converter app using iTunes File Sharing (open iTunes > your device > scroll down to file sharing and the converter app). From there, just use the converter app to convert to mobi (for Kindle) or any other format you want.
Then simply drag the epub into the converter app using iTunes File Sharing (open iTunes > your device > scroll down to file sharing and the converter app). From there, just use the converter app to convert to mobi (for Kindle) or any other format you want.