Although digital titles will automatically check themselves in on their due date, customers have the option for early return on many titles. From the Loans page of OverDrive you are be able to click "Manage loan" to find the option of returning books early.
If that doesn't work, for more details and instructions please see the OverDrive Help article, included in the link below, "How to return titles before end of lending period."
For Kindle ebooks you can log into your Amazon account and go to Content Library and select Books. For each title you have checked out there will be boxes on right and you will have the option to Return this Book along with other actions. Once you have clicked "Return this book" you will get a message on the screen that asks "Are you sure you want to return this Library Loan Ebook?" and you can click Cancel or Return this Book. Return this Book will bring back a message that says Successfully Returned.
OverDrive Help
How to Return Titles Before End of Lending Period
Return Kindle Books