Can I change the file name or delete the file from Storage after sending it through Broadcast as this will free my storage space?

You cannot change the file name nor move/delete the file which you have used to attach to a Broadcast message. GeniusOne® creates a link to the file that is attached instead of the actual file for security reasons. Therefore, if you make any changes to the file name or delete it, the parents wouldn't be able to access the file as the link would become invalid.