Please visit this article to learn what Reopen means
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.
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
**Assignments which have passed the Due Date will not have the Reopen feature available.**