If you own a Nexus or Pixel, you know what the first Monday of the month means: a new security update. It's no different this time around: right on schedule, Google has made the October security patches available to the devices it supports directly.
As usual with Google rollouts, it's probably going to take a couple of weeks for all devices to receive the update notification. If you simply can't stand waiting, just go to Settings > System > System update and hit Check for update. This should immediately trigger the download (which is in the tens of megabytes range, nothing fancy).
The aforementioned path applies to Android 8.0 Oreo, if you're still on a Nexus 6 or Nexus 9, running 7.1.1 Nougat it will be slightly different - but if you're that desperate for security updates, you probably know exactly where to find the manual check option.
If you have a Pixel or Pixel XL, your build number after updating will be OPR1.170623.027 if you're in Canada or on Project Fi, and OPR3.170623.008 for everyone else. The Pixel C gets OPR1.170623.027, Nexus 6P has OPR5.170623.007, while the Nexus 5X is receiving OPR4.170623.009. The Verizon Nexus 6 has NGI77B as the build number, while other units get N6F27M. The Nexus 9 has two - N4F27P for the LTE model, and N9F27M for the Wi-Fi only iteration.
...And know we know the reason: They wanted to include a fix for the KRACK vulnerability. https://crackberry.com/blackberry-highlights-impact-krack-vulnerability-blackberry-products
My BB Priv just got the October security update AAQ280 stamped "Oct 5" (Kernel stamped Oct 17) today (Checked at the least daily, last time 12 hours ago) So despite BB usually being very quick on the updates, they were VERY late this time...
You can sideload OTA 100% factory- locked bootloader and no root access. Root and ROM is awesome for Nexus devices though, but your claim that you need to be rooted just take manually "flash" aka adb sideload updates is totally wrong.