How can I update my iPhone from iOS 15 to iOS 16?

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support If the above Apple articles do not help, please post back with relevant information. 

If you have an iPhone 6s, 6s Plus, SE (1st generation), 7, or 7 Plus, you cannot. For those phones, iOS 15 is the end of the line.

If you have an iPhone 8, 8 Plus, or X that is still running iOS 15, you can update to iOS 16. For those phones, iOS 16 is the end of the line.

If you have a newer phone that is still running iOS 15, you can update to iOS 18. The time window has passed where you would be offered the ability to make an intermediate stop at iOS 16 (or even 17).

If you are currently running iOS 15, there's a good chance you own an iPhone 7 or 6s. If that is what you own, you cannot update to iOS 16, iOS 17 or iOS 18. The phones are too old and lack the hardware required to be updated past iOS 15. iPhone 7 was released nearly 9 years ago and iPhone 6s, nearly 10 years ago.