Google has posted the factory images and OTA Zip files that contain the September security patch fixes, so if you have a Nexus device like the Nexus 6P or maybe even an older device such as the Nexus 5, you can head to the Google Developers page where the factory images are located and download the file that matches your device. In addition to grabbing the factory images for your smartphone, tablet, or Nexus Player, you can also choose to grab the OTA Zip file to flash instead as Google has made those available as well.
If you're looking to go ahead and proceed with flashing either piece of firmware instead of waiting for the update to come through over-the-air, it's worth noting that while the latest security patch has been posted for both, a few devices, which include the Nexus 6P, Nexus 9 LTE, and the Nexus 6, only have the latest security patch software as part of Android 6.0 Marshmallow. Images or OTA Zip files for those devices with Android 7.0 Nougat are not yet available which means that while owners of those three devices will get the newest security fixes, they'll have to continue waiting on a bump to the latest system version software that includes all of the goodies which Android 7.0 introduces.
Having said that, the September security patch is important even with the lack of a bump to Android 7.0 Nougat, as it comes with changes to the media playback system where the Stagefright bug was a risk to the user. Over on the Google Developers page users will find the latest security patch update available for the Pixel C, the Nexus 6P, the Nexus 5X, the Nexus 6, the Nexus Player, the Nexus 9, the Nexus 9 LTE, and the Nexus 5. The installation process for these factory images and OTA Zip files is not as straightforward as the installation process for an OTA software update which gets pushed out to the device. You can follow a tutorial for flashing factory images that can be found here, and you can grab the downloads for either factory images or the OTA Zip files from the buttons below.