It's that time of the month—the new Android security patches are heading out to Nexus and Pixel devices. The OTA will show up on its own soon enough, but if that's not actually soon enough, the OTAs or images can be downloaded from Google's developer page right now.
Most eligible devices have builds up, but the LTE Nexus 9 is completely absent from this month's security patch. Any other month, that would be the most unusual part. However, this time there's still no Nougat in sight for the Nexus 6. Google has, however, issued a Marshmallow-based security patch for that phone. Does that mean no Nougat this month? It's all very strange. Here are the builds we've got so far.
Pixel C: NRD91D
Nexus 6P: NBD90X
Nexus 5X: NBD90W
Nexus 6: MOB31H
N9 Wifi: NRD91D
Nexus Player: NRD91D
You can get the new builds as OTA ZIPs or full images to be flashed to your device. If you don't do either, the OTA should show up in a few days.
Google has posted the security bulletin to go along with the new patches. There are 2 patch levels—October 1st and October 5th. The one from the 1st is a partial patch, and the 5th is the full patch with more critical bugs.