If it is an electronic copy, you don't need to return it at all. That copy is yours to keep!
If it is a physical item, you can return it during business hours at the Circulation Desk or you can drop it off in the book drop (located outside near the entrance of the Library). If you return it over a campus closure, you may accrue fines because the item cannot be checked in or returned until operations resume.
ILL items cannot be kept past the end of the semester in which they are borrowed. Exceptions may be made on a case-by-case basis for graduate students, doctoral candidates, and faculty members working on research projects that extend past the traditional semester.