You can return a book after opening hours. Most of the libraries at Lund University have got a drop box ("bokinkast") for books where you can return books when the library is closed! The drop boxes are emptied when the library is opening and your returned book will then be removed from your library account.
If you are uncertain whether your library have a drop box for books you can check for information on the library's website or return the book at another library which belongs to Lund University. You can also send the book by mail. See the link below for contact info, address and websites to all libraries at Lund University.