How do I get iOS 14 on iPhone 6 Plus

You can not. iOS 12.5.1 is the highest possible for that model.

You don't. The iPhone 6 Plus will not update past iOS 12.5.1. It does not have sufficient hardware to support the newer iOS.

Follow these steps exactly:

1. Open the App Store app
2. Search for and install the Apple Store app
3. Choose a new iPhone and buy it
4. Wait for it to arrive
5. You now have iOS 14