back up new phone form the cloud
how do I back up my iPhone form an iCloud save?
What you ask is confusing. Not sure what you are trying to accomplish.
You would not be able to "back up" your iPhone from an iCloud backup.
However......erase the iPhone, then you would be able to set it up and "restore" the phone again using a previous backup of the phone stored on iCloud.
Then, the phone will look like it did on the date that the backup was stored on iCloud. Hopefully, you have a very recent backup if are restoring the phone, so you won't be losing much data in the process.
Important Note.....you will not be able to downgrade the iOS version on the iPhone, if that might be what you are asking.