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iCloud basic storage plan is 5GB and it doesn't cost anything. But if you want to backup everything from your iPhone without any interruption, then instead of deleting files from iCloud, you should opt in for a high capacity storage plan like a 50GB for $1 per month or 200GB plan for $3 per month (if your device storage is higher than 50GB or if you own mulitple devices). Thus, you don't have to worry about the space.
To upgrade, refer this article --> https://support.apple.com/en-in/HT201318
If you would like to see the contents on iCloud, then go to Settings > your account name > iCloud and enable iCloud Drive. Now you will see the iCloud Drive on your home screen. Then you can view or delete files from there. But I'd recommend to go for upgrading the storage as the prices are very cheap.
If you have any questions or would like any additional information, please don't hesitate to ask.
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