@BRITNIPASCOE ...
By "categories", are you referring to assignment groups on your "Assignments" page? I ask because you mentioned "imported assignments", and that assignment group usually shows up after a course import. So, if you go to your "Assignments" index page, you should see assignment groups with the same names as those you see displayed in your "Grades" page. If you need to change the name of any of those, you'll do it on the "Assignments" index page, and then it will be changed on your "Grades" page as well.
How do I add an assignment group in a course? - Instructure Community (canvaslms.com)
See the section on "Manage Assignment Group" within the Guide I've linked above.
Hope this helps!