How to update ios12.3 to ios15
iPhone 6 Plus, iOS 12
The iPhone 6 Plus can be updated to iOS 12.5.5, no further.
The iPhone 6 Plus can be updated to iOS 12.5.5, no further.
In your initial post, you stated it was at iOS 12.3. Your screenshot shows it is at iOS 12.5.5.
As in my initial reply, it can not be updated further.
Did you possibly use a beta version on the iPhone? If so, see: How to uninstall iOS or iPadOS beta software - Apple Support
Also check at: Settings - General - iPhone Storage- check her for an unused iOS update file.
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