When you delete or deselect a file it is removed from your backup configuration and marked for deletion on the Online Backup servers. Online Backup keeps a copy of that file in older backup sets just in case the deletion was accidental. Those older backup sets are kept for 30 days and then deleted. To restore a file you have accidentally deleted, go to one of these older backup sets, before you deleted the file. Restore the file from here and include it in future backup sets.