To use the drag-and-drop calendar, go to the calendar view of a table.
Now when you put your mouse over the mini calendar icon
in front of the task name, the cursor will change to the move pointer, on Windows machines, it's usually a cross with arrows. On Macs, it's a little hand.
Click and hold this icon to drag the task to another day on the calendar. The due date for the task will automatically update to the day you dragged the task to.