To reschedule an already scheduled email campaign, click on one of the email campaigns from the list and select the Schedule Email tab in the detailed view.
There, click on the Actions menu of a schedule and select Reschedule (or Send now / Delete)
If you click on reschedule, simply select the new date and exact time at which the email should now be sent. You can edit these settings as many times as needed.
Lastly, make sure to save the changes you have made.