As long as your instructor has the assignment options set to "Allow Reopening" and it is before the due date, you will be able to Reopen your attempt a few ways. First, check to see if you can reopen the assignment by going to the Assignment Overview. It will look like this:
From the Assignment Overview page
When on the Assignment Overview page, the second section named Attempts will show you information about all of the attempts you have for the assignment. On the right side, you should see the Reopen option.
From the Experience Overview page
When on the Experience Overview page, you should see the Reopen button across the top regardless of which