How do I update my iPhone 6 to the latest iOS version like for example iOS 14?

You purchase a newer iPhone that supports the newer iOS. It has to be a 6s or above. However, given the support for iOS 16 coming out this Fall, you would be better off purchasing an iPhone 8 or above, since the earlier devices will not support iOS 16. That way your device will be supported longer.

You already have the answer in the post you responded to: You DON’T with that phone. You will have to get a newer phone, a iPhone 6S or later. And, if you want iOS 16 when it comes in the fall, you will need an iPhone 8, SE 2nd edition, or later.

Unfortunately, an iPhone 6 is unable to receive updates to the latest iOS, currently iOS 15.5. iOS 12.x is as far as it will go.

An iPhone 6 isn't capable of 13 or above. You need a newer unit.

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So how do I get the iOS 15 or 14?

You get this by buying a new iPhone. My recommendation would be an iPhone 11 or higher, if possible.