I want to update my iPhone 6 Plus to iOS 14

Nice picture! .. but you cannot update an iPhone 6 Plus past iOS version 12.x. If you need to run apps that require a newer version of iOS, you will need to get a newer iPhone model.

nazirul ~ Apple offers many newer iPhone models which will run iOS 14:

iOS 14 and iPadOS are compatible with these devices

...found by searching Apple's Knowledge Base.

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