Go to Dashboard=>Tool=>WP-DB Backup > Settings > Number of backups in the admin dashboard. Enter Number of backups to store on this server.
Past this limit older backups will be deleted automatically.
The disk that your backup is saved on doesn’t have enough free space? Backup disk is almost full? Low disk space for backup? Backup failed due to lack of space? As you may set up a schedule to automatically do backup daily or weekly, and the size of disk space is limited, so your backup disk will run out of space quickly or someday. It is a real pain to manually delete old backups. Don’t worry about it. WP Database Backup makes it easy to delete old backup files automatically.