If your assignment has a submission deadline:
Did the submission deadline pass? Answer: No
You can change your submission before the deadline for submission has passed.
To change your submission click on your class and then on the assignment you wish to change.
You can now delete or unsubmit the submission by clicking Delete Submission and upload a new one or click Change to edit the text or change the file.
Did the submission deadline pass? Answer: Yes
After the deadline has passed you are no longer able to change your submission, in this case reach out to your teacher.
If your assignment does not have deadlines:
You will need to contact your teacher and ask that you be 'restarted' in the assignment.
After you have been restarted you will need to remove your file or link and then upload or attach a new submission.