Changing schedule on a single device:
Select the new schedule and click "Add New Schedule"
Select the Calendar Icon under First Date and then choose when the backups first started coming through. It could be 1 week or several months ago so you don't lose your history. Then click Save. Then submit the changes and the schedule will be adjusted.
The same applies to editing the backups in bulk but you will not have a schedule option showing that you have to remove. The system will overwrite the backups you selected with the schedule you choose.