If you have been using iCloud for a long time now, chances are that you may incur storage cost sooner or later. iCloud backup files take too much space on your iOS device as well as your cloud storage, so, you must delete some old iCloud backup files, otherwise, you have to pay for extra storage.
Follow the steps, you can delete old iCloud backup files on your any of your iOS device.
Tap Settings > iCloud > Storage & Backup > swipe iCloud Backup to ON > tap Manage Storage in the same window. Tap on the backup file you want to remove from iCloud > tap Delete Backup.