How can I update my iOS 15 to 16 please

This sounds as if you might have an iPhone 6s, iPhone 6s Plus, iPhone 7 or iPhone 7 Plus.

if that is the case, you cannot update to iOS 16.

If you have an iPhone 8, iPhone 8 Plus, or iPhone X, you can update to iOS 16.

Tap Settings > General > Software Update. If your phone is qualified for an update, the update option will appear.

Go to Settings > General > Software Update and see if there is an available update for you. If nothing appears, you are on the latest iOS software update for your model, which most likely is iOS 15.8.3. iOS 16 is compatible on the iPhone SE (2nd generation), iPhone 8 and newer. If so you should start looking into buying a newer iPhone that can run newer iOS software.

If you are on iOS 15, you most likely have an iPhone 6s, iPhone 7 or first generation iPhone SE. For all three phones, they lack the hardware required to update higher than iOS 15. As such, if you need iOS 16 or higher, then you'll need to retire your old iPhone and buy a new or newer more modern iPhone at this point.