How do I update my iPhone to iOS 15?

Your tag line says that you have an iPhone 6.

If that is the phone you are trying to update, you cannot update it to iOS 13, 14, or 15. iOS 12 is the end of the line.

Is that the phone you were asking about?

iOS 12.5.7 was the last update for the iPhone 6.

There are no more updates available for the iPhone 6.

If you want to run iOS 15, you need an iPhone 7.