I want to update my iOS 12 to 14

If you're referring to an iPhons 6 Plus or earlier, it's not capable of 13 or above; an unit on 12.4.2 through 12.5.6 will fall into this category. Otherwise, use a computer or the Software Update function in Settings to install 15.6.1.

It's no longer possible to update to exactly 14 regardless of what model it is.

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