Titles you've borrowed from the library will automatically be returned at the end of their lending periods. However, if you finish eBooks, audiobooks, or streaming videos before then, you can return them using OverDrive for iOS (to free up space on your library account).
How to return eBooks, audiobooks, or streaming videos
From your app bookshelf, tap and hold a title to display its return options. Note: If you're using the legacy version of OverDrive's app, tap the icon to display the return/delete options.
You can Return a title to the library, which also deletes it from your device. You can Delete a title from your device, but you will still have it checked out to your library account.