I need your help to update my iPhone

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Tap Software Update

If an update for the iPhone 6 is available, it will appear. If you see that the iPhone is running iOS 12.5.6, the iPhone 6 has been updated as far as it can go. No more updates are available.

The iPhone 6 cannot be updated beyond iOS 12.x.x. It lacks the components necessary to run newer software. If you want newer software you will need a newer phone...