To become the beta tester of Android Pie for Nokia 6.1 Plus, you have to register in the Beta Test program at Nokia Mobile Beta Lab web page. After you sign in, you’ll have to validate your device by entering the IMEI number, your operator name and then ask for OTA update. Nokia Mobile is currently testing version V3.25B of the Android 9 Pie.
If you don’t want to participate in the beta test, you can always unregister at the same page you registered.
Please, do tell us how the process went for you since we can’t get our hands on 6.1 Plus, unfortunately.