Locate the User (see how do I find a User if you're unsure how to do this).
Click on their Username to view their profile.
Click on the My Activities tab.
Click on the Due Date link in the Due Date column for the Activity you’d like to amend.
In the popup window, use the date picker to specify a new due date. Although the note is an optional field, we would recommend adding a reason for adjusting the due date so that other Administrators are able to see why the adjustment was made.
Click on the Save Changes button within the popup to save the new due date.