How to update my iPhone 6 to iOS 15.7.9?

Your iPhone is too old and would not be compatible with the newer software updates.

Click ▶︎ iPhone compatibility with iOS 13 for the complete list.

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The iPhone 6, released by Apple over eight years ago, cannot be updated beyond iOS 12.5.7.

It is not possible to update the iPhone 6 to iOS 15.x.

The best that the iPhone 6 can do is iOS 12.5.7

If you want to run iOS 15 or iOS 16, you will need a new phone.