UPDATE (1/12/16): Many users are reporting that the 8.7MB update is live for them, so expect to receive the update on your Nexus device shortly.
As Google had previously promised, they would release monthly security updates for Nexus devices. So far, they have been true to their word and we have been greeted with the January security update, available in the form of an OTA update or factory images, for Nexus devices. You can download these images from the Google Developers website, or just wait for an undefined period of time for the update to come the easy way to you (read: OTA). We are looking at build MMB29P for the Nexus 6P and 5X, MMB29S for the Nexus 5, Nexus 6 and Nexus 9 (Wi-Fi and LTE, models), MMB29T for the Nexus Player, and MMB29O for the Nexus 7 (Wi-Fi and Mobile models). This update has also made a few appearances on carrier support websites, such as Sprint and Verizon.
Included in this update are several security fixes (listed below). There is no word yet on what (if any) bug fixes are included in this release, but we certainly hope there will be some much needed fixes included.