How can I get iOS 13.0

Are the iPhone 5 and iPhone 6 using the same Apple ID? Some developers will allow older models to install older app versions if the app was originally purchased under the same Apple ID when the older app version was available. However, this is all up to the app developer so you will want to reach out to Nord VPN as it is their decision and app.

How to contact an app developer - Apple Support

iPhone 6 cannot update past iOS 12 because the hardware is not compatible with iOS 13 or later.

Then ask Nord VPN how to download it, this has nothing to do with Apple.