To reschedule an action item, go to Menu > Action. Tap the Action Items icon. View the list of items. Sort and filter as needed to find the one you are looking for.
Tap the action item you want to change. Update the description, date and time as needed. You will receive a new calendar event email with the updated information. To add it to your calendar, open the attachment in the email you receive and save. Delete the old event from your calendar.