How do I update my iPhone from iOS15 to iOS 16?

If you have an iPhone 6s, 6s Plus, SE (1st generation), 7, or 7 Plus, you don't.

Those iPhones can run iOS 15 but cannot be updated to iOS 16 or higher. You need at least an iPhone 8, 8 Plus, or X to run something higher than iOS 15.

Your phone model must be on this list to upgrade to 16:

iOS 16 and iPadOS 16 are compatible with these devices - Apple Support

To identify your phone model (which you have not shared with us):

Identify your iPhone model - Apple Support

If you don't have the option at Settings > General > Software Update, then it is probably because your device is not capable of running iOS 16. Have no idea which iPhone you are using though, so it is impossible to know.