The first step in returning your eBook early is identifying which eBook reader software you used to read your eBook. and the vendor that supplies the book (e.g. Ebook Central, EBSCO, etc.).
In EBSCO, for instance, you can check a book back in before the check-out period is over if using Adobe Digital Editions (learn how here).
To find out what the policy or procedure is on other book sites, search the vendor's support pages for "check in" or "return". In many cases, when you check out a book, the necessary information will be provided.