Recently Turn It In (TII) updated their assignment submission box for Canvas. This is a bit confusing for students looking to resubmit their assignments. Given the assignment allows re-submissions, a simple click of the upload arrow to the right of the original file upload will prompt the student for permission to resubmit.
If you still can not submit, the assignment does not have re-submissions allowed. If they should be allowed, please contact the instructor to let them know immediately. For any other questions or issues, please contact IT.