Assignment due dates can be updated in Forum. Note that you must be in the Academic Admin role in order to change due dates.
How to update assignment due dates in Forum
To update the deadline:
Open the assignment page
Click on the button on the top right titled Edit Due Date
Set the new due date, and click Save
One restriction on due dates is that you cannot set the due date to a time that has already passed. However, you can edit a due date for assignments that already have submissions.