If you need to adjust the due date on an assignment or decide to push it back to give students additional opportunities to complete the assignment, you can do so in your Assignments tab.
Click on the Classes tab, choose the class and click "View Class".
Select the Assignments sub-tab.
Click on Options next to the assignment that you wish to adjust.
Select Settings where you can change the due date to any date as long as it has not already passed.
Note: Students will get an email notification once you save the assignment with the new date.