How to remove an old backup from iCloud?

See the following from Manage your iCloud storage - Apple Support

Delete backups and turn off iCloud Backup for your device

You can delete old iCloud backups for the device you're currently using, or older devices you may not use anymore. iCloud backups are available for 180 days after you disable or stop using iCloud Backup.

iPhone, iPad, and iPod touch

1. Go to Settings > [your name], then tap iCloud.
2. Tap Manage Storage > Backups. 
3. Tap the name of the device whose backup you'd like to delete. 
4. Tap Delete Backup > Turn Off & Delete.

When you delete a backup, it also turns off iCloud Backup for your iPhone, iPad, or iPod touch.

Open up iCloud from the Settings app. Then click or tap Manage Backups to see all of the backups that are currently saved to your iCloud account. Delete the ones you don't need any longer.

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