Users can modify a due date by using the edit task feature or by using the drag and drop feature in the calendar application.
From My Day:
      1) Right click and select Edit Task. 
      2) Change the Due Date and click SAVE. 
From Tasks:
     1) Double click on the task to pull up the edit menu. 
     2) Modify the Due Date and click SAVE.
From Calendar:
1) Right click on the task and select Edit Task.
OR
2) Left click on the task and drag & drop it into the day that reflects the new due date.
Related Links: 
    - Set Due Date
