Backing up iCloud

See “If an alert says that there's not enough iCloud storage“ at If you can't back up to iCloud - Apple Support

Foxy595 Said:

"Backing up iCloud: it won't let me backup my iPhone and is saying I do not have enough storage, but I have 1.3 GB out of 5 GB. What can I do?"

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Clear Space.

Troubleshooting iPhone Storage:

Internal storage cannot be upgraded. It is what it is. So, the maximum internal storage capacity never increases, even if you upgraded your iCloud plan. Learn More here: What's the Difference Between Device Storage and iCloud Storage? Apple Support

So, what to do...

Clear Space Using my User Tips:

• How to Clear Space from your Mobile Device, and What to Clear: - User Tip. You may have too much installed, and need to clear space.

• How to Clear "Other" in your iPhone's Storage - Use Backup - User Tip. Caches would be being used for remembrance of application use. It is essentially the same for an iPhone. You'd backup your iPhone, and then restore your iPhone from the backup you've just created. All System Data will delete.