Once logged into the Control Panel in the web app, click on the device you wish to delete from the left sidebar > then click on Settings > then click Delete This Device, a pop-up will ask you to confirm the delete. Please note that deleting a device removes all its backups and files and frees up any space the device's backup took up.
It can take up to 12 hours for this to take effect. However, in most instances it is almost immediate.
If you are removing a device and its backup with the intention of starting over, make sure you uninstall and reinstall the application so that everything works smoothly.