Earlier in the year Google announced that it would be starting monthly security updates for its Nexus devices to keep them up to date on the latest security patches and give users a little more peace of mind. Those monthly security updates began back in the first week of August, and will continue to carry out going forward. As of yesterday the most recent build of these monthly security updates were starting to arrive for some users on T-mobile including the current Nexus 6 and Nexus 9 tablets, as well as the Nexus 4, Nexus 5, Nexus 7 2013 model and the Nexus 10. This would bring users forward to build number LMY48M except for the Nexus 6 which would be receiving its own update build labeled as LYZ28K.
Usually when carriers start mentioning the arrival of over-the-air updates to a new software build, Google isn't too far behind with the factory images of that software, and as of this morning Google has posted the factory images for the latest Android firmware build as part of the new monthly security updates. As stated before this is for build LMY48M, unless you use the Nexus 6 on T-Mobile which has its own build coming from the carrier. Keeping this in mind, you don't want to flash the factory image of the LMY48M build to your T-Mobile Nexus 6 device as it could mess with it and cause something to work improperly.
For anyone else, the factory images can be found via the Google developers page in the source link below. You'll find every link there for all the devices listed above save for the T-Mobile model Nexus 6. If you're unfamiliar with flashing the factory images you may simply want to wait for the over-the-air updates to come through as they should be rolling in shortly now that the factory images are up. Additionally, downloadable OTA links will likely start appearing soon too which will give users another way to flash the new software build to their device if you prefer to wait for those. As this is simply a security update users shouldn't expect much in the way of new functions but having up to date security for your device is always a good thing.