How to update my iPhone 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. 

Not possible if you have an iPhone 6 or iPhone 7.

Possible if you have an iPhone 8 or higher numbered model.

Tap Settings > General > Software Update