How can I update my iPhone with Apple TV

You can't update your iPhone with an Apple TV but I also see that you're using an iPhone 6, unfortunately, iOS 12 is the latest version of IOS for that model. In conclusion, the phone is too outdated for newer updates. You still will receive security updates unless you're trying to update your Apple TV, Use the remote to navigate to Open Settings on Apple TV. Go to System > Software Updates and select Update Software. If an update is available, a message appears. Select Download and Install to start downloading. During the update process, don't disconnect your Apple TV. Have questions, simply reply