In most cases, each device will only have one backup, which is the most recent backup for that device. Deleting data off of your device will result in the next backup being smaller and will overwrite that last backup automatically reducing the amount of storage being used in iCloud.
You can delete that backup, but when your device backs up again, it will be the same size as the one deleted. You can also choose what data you want to be saved in the backup file. See the information here for reducing the size of your iCloud Backup and additional information about turning off iCloud Backups.
Manage your iCloud storage - Apple Support