How can I update iOS 12.5 to iOS 14 or 15

As already mentioned the iPhone is too old and does not Qualify for iOS 14 or iOS 15.

Only option to run iOs 14 or iOS 15 is the purchase a newer iPhone or New iPhone that is already running either of the two iOS

You can't. An iPhone 6 or any other unit on any version of 12.5 isn't capable of 13 or above.

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