Let's say you have six tasks you'd like to set for completion tomorrow. Two of these tasks are scheduled for today and the other four have not yet been scheduled.
Simply press on one of these six tasks and watch the check boxes change into circles.
Now tap on the other five tasks. Note that a filled-in circle indicates that you have selected that task, and the number "6" appears in the header showing you how many tasks you've selected.
Now tap the calendar icon at the top to display the date & time selection dialog box.
Then tap on the "Tomorrow" icon, and all of your selected tasks will be scheduled for tomorrow.
(Note that if you wanted to set a common date and time for all six tasks, you would need to press the "Pick Date" or "Today Sometime" icons)