The old canvas layout had a option where you could add a file after you have already uploaded one but now that dose not exist.
As a student, you should be able to resubmit an assignment:
However, the course instructor may have edited the availability dates. It's like their way of cutting off the assignment for their students. As long as the assignment is "available" the re-submit button should be there!
It would be great if you could double-check those dates (you'll find them inside the assignment details). Please let the Canvas Community know if you have any follow-up questions.
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