Some settings have been disabled because the assignment is in a closed grading period.
How can I change this one so that I can grade their work?
I'm guessing here a little because we don't use grading periods. But, my understanding is that the due date of the assignment is what control which grading period it falls under. So you might try changing the due date to see if that has any impact. Like I said I'm guessing and if this works I'd consider it a hack and not a real solution.
My workaround for one of my teachers with this problem was to make him an admin over his subaccount in Canvas. So admins can edit due dates after grading periods have passed, but regular teachers cannot. I can't speak to the other losses of functionality but I was totally unaware that editing an assignment becomes impossible after a grading period has passed!
EDIT: I found out that I forgot to make the Closing Date of the Grading Period extend to the end of the school year, so I just did that and all my problems went away. I understand closing dates for grading periods and teachers/schools should know or have a policy about how long teachers have to access and change grades, and then have Grading Periods set up accordingly.
Yeah, the ability to change the due date is not available to teachers after the grading period has closed. You should talk to your Canvas Admin about extending the Closing date for the grading period, it's possible that they missed setting that up to give teachers enough time to grade things.