NOTE: You can only extend a due date if the Course already has a due date set. 

Extend a Course due date via Admin View > Content > Select ‘Courses’ > Select a specific Course > Select the ‘People’ tab > Select/deselect the User(s) you want to extend the due date for >  Select ‘Extend Due Date’ from the ‘Perform a bulk action on selected people’ dropdown > In the popup window, select None, A Time Span, or A Specific Date from the dropdown > Select either the number of Days or Weeks (for A Time Span) or select a due date on the calendar (for A Specific Date) > Select ‘Update.’ 

Changes to a due date may take a few minutes to appear for the selected people, and you may need to refresh the page to view it. 

Please note, if the user already has a due date set, choosing to extend a due date, then selecting ‘None’ will not remove an existing due date.