Can I update my iPhone 6 from iOS 12.5.7 to iOS 14?

The iPhone 6 cannot update further then iOS 12.5.7 due to hardware limitations. That iPhone is too old to update to iOS 14. If you need to run iOS 14 or higher you will need to get a newer iPhone I am afraid

FYI, if you can obtain use of an iPhone 6s, it’s capable of running iOS 15.

That said, would STILL suggest a “newer” model; one at least capable of running “today’s” iOS 17.

(Meaning at-least an iPhone Xs)