iCloud & iCloud Photos · Messages, Mail and device backups in iCloud
How do I delete old iPhone or iPad backups from iCloud on my Mac?
Short answer: In System Settings → your name → iCloud → Manage → Backups, select the old device and click Delete. Only delete backups you'll never restore from; deleting one also turns off iCloud Backup for that device. iCloud Photos isn't affected.
Open System Settings, click your name, choose iCloud → Manage, then Backups. Select the device whose backup you want to remove and click Delete (or the minus button). The space is returned to your iCloud plan.
Deleting a backup turns off iCloud Backup for that device if it's still in use, and you can't restore that device from it any more — only delete backups of devices you've replaced or no longer need. Photos stored in iCloud Photos aren't part of device backups and aren't touched. For backups stored on the Mac itself, see deleting old iPhone backups from a Mac.