If you only have to extend the Due Date of a particular assignment and plan to leave all other settings the same, the simplest approach is to do this from the Drag/Drop Calendar.
Click on the Calendar
Select Entire Class or Individual
Using the drop-down menu, choose whether or not you want to change the due date for the entire class (the default) or for an individual. If you're changing the due date for an individual you may have to click Add Individual and then add the student in question to select them.
Click and drag the assignment to the new due date