A course backup file (.mbz) may be restored from within any existing course for which you have permission. During the restore process, you will be given the option to restore as a new course or into an existing course.
Moodle's file import tool can be used to bring over files and activities between courses on Moodle. It does not transmit most user or grade data. When you're setting up a new version of a course you have taught previously, you may prefer to use the import function unless there is a particular reason to use the restore function. See Importing Course Materials from Past Courses for instructions on using the import function.