Delete file offline If you have a backup plan that you are no longer using and you have no need for any of the data that has been backed up, you can delete the data from storage using the Storage tab. Then you can either delete the backup plan or remove the schedule so it does not run.
If, on the other hand, you are looking to free up space on existing backups, then do what Ivan posted and check your retention - you may be keep more data than needed. Feel free to reply with your retention needs and a screenshot of your retention settings if you would like us to review here.