Changing schedule on a single device: 

Start by clicking on the X to remove the existing schedule.

Select the new schedule from the dropdown and click save. 

To edit the date range click on the new schedule and backdate the job. It could be 1 week or several months ago so you don't lose your history. Then click Save 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.