Need download beta iOS 14

1. Back up your device using iCloud or your computer.
2. Plug your device into power and connect to the internet with Wi-Fi.
3. Go to Settings > General, then tap Software Update.
4. If you see more than one software update option available, choose the one that you want to install.
5. Tap Install Now

iOS 14 beta is not available. An iPhone 6 Plus cannot be updated past iOS 12. You need a newer phone.