I wang to upgrade my iOS 12 to iOS 14

Hello ~If your iPhone is a 6 plus it cannot be updated to iOS 14 . It is already at the highest iOS version your device can support. If you need an updated version of iOS you need a newer iPhone. Go to black line at the top of this page and click on iPhone to see new choices. If you are considering a second hand device please take a moment to read this information:

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Also:

The All Too Common SAD Reality of Buying a Used iPad/iPhone

~Katana-San~

You can't. An iPhone 6 isn't capable of 13 or above.

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