Rescheduling incomplete tasks
Some tasks are not as complete as they were scheduled to be. 
Click the Reschedule toolbar button. 
Choose a date and time to reschedule the incomplete tasks, and whether to update all tasks or just the selected ones. 
The incomplete tasks move, and the whole schedule updates, so that work can continue from here. 
Note that this only moves tasks with incomplete work that was scheduled for before the date you specify; tasks in the future aren't affected.