You can't return a book on the SFPL Overdrive site, nor on the Kindle itself.
But you can return books on the Amazon website, under Your Account->Manage Your Content & Devices.
If you click the "..." button next to a checked-out book, you'll get a pop-up menu.
|I added that circle to "Return this book."|
Select "Return this book," and after a while, it'll disappear from your checked-out shelf on the Overdrive site, freeing up a slot for another audio or Kindle book!
Next, I'll show you how to check out and use an audio book on Overdrive.