So I am confused one how you update this iPhone 12

To update just follow the steps in the support document you posted from. Settings->General->Software Update.

If you have an iOS version that supports screen sharing (15.1 or later), then you can tap on the Screen Share icon during a FaceTime Call to share your screen. Link->. Share your screen in a FaceTime call on iPhone - Apple Support

Follow the steps in the Related Article linked above.

If you have questions from that, or encounter error messages or issues with that, please post them.

Hello~ What happens when you try to update? Take a look here…

If your iPhone or iPad won't update - Apple Support

If you can't make or receive a FaceTime call on your iPhone or ...

~Katana-San~

The link at the top of the page has the information on how to update your phone.