How can I update my iPhone 7 Plus to iOS 16.3?

Your iPhone 7 Plus no longer receives the latest iOS software updates and cannot be updated past iOS 15.8.3. You need an iPhone X or later to run iOS 16. Since you should probably look into a new iPhone that can run a more up to date iOS, I would suggest an iPhone 15 Pro or later as they support Apple Intelligence, and the latest iOS 18.

Sorry, the iPhone 7 Plus cannot be updated beyond iOS 15.8.3. That is the last possible update for it.