Unfortunately, ebooks can not be renewed. They can only be checked out again after being returned. If you don't want to wait for them to automatically be returned after the 14-day checkout period, you can return them early using the Manage Your Kindle link on Amazon.com when signed in to your Amazon account.

http://www.ropl.org/index.php/online-resources/audiobooks-a-ebooks/4195-transferring-ebooks-to-a-kindle