Earlier in the year Google announced that they would be doing monthly security updates for various Nexus devices going forward, and just yesterday we saw the latest software build of those security updates hit the developer page as factory images for a multitude of different Nexus devices. This at the time included the factory image for build LMY48M for the Nexus 6 models (all but T-Mobile and Project Fi), the Nexus 7 2013 WiFi only model, the Nexus 4, the Nexus 5, the Nexus 9 for both WiFi and LTE, and the Nexus 10 tablet. T-Mobile and Project Fi models of the Nexus 6 were left out because they have their own special build due to certain factors.
As of today though Google has posted the latest security update factory image for the T-Mobile and the Project Fi models of the Nexus 6, which are labeled as build LYZ28K for the T-Mobile model and build LVY48F for the Project Fi model. Now that these factory images are up those who don't want to wait for the software to be pushed to their device over-the-air can flash the factory image instead, although we only recommend this if you know what you're doing as one tiny misstep can damage your device.
In addition to the two new Nexus 6 models which have their new factory image builds, Google has also posted the factory image for build LMY48M for the Nexus Player, bringing it forward with the latest security fixes and patches like all the other devices. Lastly, Google has also posted the latest factory image build for the Nexus 7 2013 LTE model today, meaning both WiFi and LTE 2013 Nexus 7 users can flash the factory image if they feel so inclined. For the LTE model of the 2013 Nexus 7 the build will be labeled as LMY48P. As stated above you can find the correct factory images for your device over at the Google Developers page where all the software links are listed and categorized off by device. The latest build will be at the very bottom of each category. If you're uncomfortable with flashing factory images or if you have never done it before, you can simply wait for the OTA update which shouldn't be too far off.