How do I update my iPhone 6?

An iPhone 5s, iPhone 6, or iPhone 6 Plus isn't capable of 13 or above; they require at least 2GB of RAM. To install 12.5.7, use a computer or the Software Update function in Settings.

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Your iPhone 7 has reached the end of the road for the latest iOS updates and cannot be updated past iOS 12.5.7. If you want to update to that, you can do so in Settings > General > Software Update. Otherwise, you’ll need to purchase a newer iPhone that has the hardware capabilities to run newer iOS.