How to update my iPhone 6

You probably already have the latest update for the iPhone 6, which is iOS 12.5.7. There are no more updates after that.

Tap Settings > General > Software Update to check

Your tag line indicates you have an iPhone 6 running some version of iOS 12. An iPhone 6 can only run the latest version of iOS 12. It cannot run iOS 13 or higher. If there are some apps that require more recent versions of iOS, you will have to do without them, or get a newer phone.