Only Adobe eBooks can be returned early. To early return an Adobe eBook:

If you are using Adobe Reader 8...
Open Adobe Digital Editions.
(1.) Select the "Library View" icon (in the upper left corner). The Adobe Digital Editions library is displayed.
(2.) Under the "Bookshelves" heading, select "Borrowed".
(3.) Click the desired eBook.
(4.) Click the "Item Options" arrow (in the upper left corner of the selected eBook).
(5.) Select option to return the item.

If you are using Adobe Reader 7...
(1.) Open Adobe Reader 7 and go to File > Digital Editions > My Digital Editions
(2.) Locate the cover image of the eBook you wish to return early. Right-click on the cover image and choose the 'Return to Lender' option. ('Document Expiration' dialog box will display.)
(3.) In the 'Document Expiration' dialog box, click 'Return to Lender' option. The Adobe eBook will then be removed from your eBookshelf.

If you are using Adobe Reader 6...
(1.) Open Adobe Reader 6 and go to File > My Bookshelf
(2.) Locate the cover image of the eBook you wish to return early. Right-click on the cover image and choose the 'Return to Lender' option. ('Document Expiration' dialog box will display.)
(3.) In the 'Document Expiration' dialog box, click 'Return to Lender' option. The Adobe eBook will then be removed from your eBookshelf.

Or, refer to the following FAQ at eBooks.realtor.org (at eBooks Help "Borrowing Help"):

Can I return my items early?
http://ebooks.realtor.org/FE2D0A48-EE48-41CC-9BDE-F00BFAED9ADE/10/214/en/Help-Policies-Borrowing.htm#question-124