Factory and OTA images have gone live for the September Android Security patch for Nexus and Pixel devices, and this is the same update for security fixes that Google started pushing to Pixel and Nexus devices as of just yesterday. This means that if you don't want to wait for the security update to actually come to your device naturally which is over-the-air, then you don't have to wait as you can simply click one of the buttons below to go straight to the download pages for the new patch, you just need to pick whether you want to install the Factory Images or Full OTA Images. Both are basically the same, so it really shouldn't matter which one you pick.
The images are available for every Pixel or Nexus device that is still supported by Google except for the Nexus Player, which the image hasn't shown up for yet. As for the devices that do have images available this includes the Nexus 9 in both Wi-Fi only and LTE models, the Nexus 6, the Nexus 5X, the Nexus 6P, the Pixel C tablet, the Pixel, and the Pixel XL.
Security updates aren't terribly large so the downloads won't take too long to finish if you're downloading them through your phone's browser, which means it won't eat up a lot of data from your wireless plan if you don't have unlimited 4G LTE. Some things to keep in mind are that even though this is a small update, around 32MB, you still might want to be connected to Wi-Fi and you do want to have a fair amount of battery life left, as you don't want the phone to power off in the middle of this process. You also want to ensure that if you're not already aware of how to flash an image to your device, you follow the step-by-step instructions that Google has laid out for users on both the Factory and OTA image pages. These instructions are pretty straightforward, but it never hurts to brush up on some of the details surrounding this sort of process. If you're worried about damaging your device, then you can just wait for the update to show up on its own, or you can go into the phone settings and see if it's available to download by checking manually for it.