If you need to update the metadata (including the description), you need to email iBookstore support. Even though it seems like you should be able to modify these from iTunes Producer, you actually can't. See the iBookstore FAQ for help on this. It's addressed directly in that document. You need the ISBN, title, and Apple ID (not your email, but the number assigned by iTunes to your specific product. The Apple ID is found on the product page in iTunes Connect).
Otherwise follow Aaron's advice. You CAN update the cover and ePub files from iTunes Producer, just by looking up using the ISBN.