How do I update my iPhone 7 to the latest iOS version?

Go to settings - general - software update and start there. An iPhone 7 cannot update past iOS 15. If you need a newer iOS version, you will need a new phone.

You do not update an iPhone 7 to iOS 18.* (the latest iOS version), because you can't.

You can update an iPhone 15 to the latest release of iOS 15 – currently iOS 15.8.3. You cannot update an iPhone 7 to iOS 16 or higher.