Download IOS 14.0

You don't.

It is too old.

If you are running something older than iOS 12.5.1, you can update to that by going to settings>General>software update while connected to WiFi with at least 50% battery charge.

You can’t with an iPhone 6 Plus. It can’t be updated past iOS 12.5.1.

Your device is to old for software updates.

If you need a newer iOS version, then you will need a newer phone.