The Galaxy Note 9 Pie update should also be available for those on Android Oreo, since the stable Galaxy S9 update was also released for non-beta users. However, it remains to be seen just how widespread the release will be, as even the Galaxy S9 Pie update is still available only in a few countries. Samsung did say Android Pie for the Galaxy S9 and Galaxy Note 9 would be out in January, though, so it's possible the official rollout may begin in earnest in the next few days.
The software version for the stable Pie update is N960FXXU2CRLT and it includes the January 2019 security patch. To download it on your Galaxy Note 9, tap the Download and install option in the Software update section of the phone's Settings app. Once you've updated, do check out what's new with Android Pie and also tell us if you're seeing the update in a country other than Germany.
