When you finish an eBook early, you have the option to return the title to your library.
Returning a title
Open Adobe Digital Editions.
Select Library from the upper-left-corner if you don't already see the view below.
Right-click on the title you'd like to return. Click Return Borrowed Item.
A dialog will appear asking if you're sure you want to return the eBook. Click Return to confirm.
Note: You can only return one title at a time. If the option is greyed out, make sure that you've only selected one book.
That's all there is to it. If you feel that a shorter lending period might be better for you, you may be able to adjust how long you borrow a title using your library's website.