Why won't my iPhone 6 update to iOS 14?

This is a user to user forum, Apple is not here.

You have an old phone, it cannot support a newer operating system.

Threatening to buy an Android has no merit here, you're welcome to purchase whatever phone suites your needs.

And, like Apple phones, older Android phones won't t update to the newest operating system either.

jai31 wrote: "I need to use this app"

Then get a newer iPhone model which is compatible with iOS 14 required by that app – there are many such iPhone models to choose from:

iOS 14 and iPadOS are compatible with these devices

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You cannot update an iPhone 6 beyond iOS12.5.4. It is too old to run anything later than that.