To free up space in iCloud, take a look at this Apple article, see -> Manage your iCloud storage -> Manage your iCloud storage - Apple Support
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Make more space available in iCloud
If you run out of iCloud storage, your device won’t back up to iCloud, new photos and videos won't upload to iCloud Photos, and iCloud Drive, other iCloud apps, and messages won't stay up to date across your devices. And you also won't be able to send or receive emails with your iCloud email address.
You can free up storage in iCloud by deleting content that you're not using:
Before you delete anything, you might want to archive or make copies of the information that you store in iCloud.
Reduce the size of your iCloud Backup
When you back up your iPhone, iPad, or iPod touch to iCloud, the most important information on your device is backed up automatically. Learn more about what iCloud backs up.
Many apps automatically back up to iCloud after you install them. You can decrease the size of your iCloud Backup and free up space in iCloud when you stop backing up apps that you don't use or delete old iCloud backups.
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