Nexus 6 on AT&T updated with Google's January Android security patches
Nexus 6 owners on AT&T are receiving an update. Sized 26MB, it has a NBD91W version number and delivers Google's Android security updates for January to the smartphone.
There aren't any other changes in the update, but this doesn't mean you should skip it. Android security updates seal security vulnerabilities in the operating system's code that can be exploited by attackers for malicious purposes. You can learn more about these exploits by following Google's Android security bulletin. Keeping devices updated is good practice both on manufacturers and users' behalf.
If you haven't received the update yet, you can do it manually from the handset's Settings menu.
source: AT&T via GSMArena