Updates on Nexus devices are usually fast coming and before other phones, but such has not been the case for every single model of the Nexus 6, Google's latest entry in the Nexus smartphone lineup. The AT&T model of the device has been waiting on the updated software for Lollipop, currently Android 5.1.1, for some time now while other models have already received it. Recently however, AT&T had made it known to customers that the Nexus 6 would be due to get the update to Android 5.1.1 sometime in the very near future. That update has started to hit AT&T device owners as of today.
With users beginning to see the status bar prompt to download and install the update software package, AT&T Nexus 6 owners should see their device move up to build LMY47Z. The only unfortunate thing is that some users may not see the update today, which is likely since updates are usually rolled out in batches. If this is the case, AT&T customers should all end up getting the update over the span of the next few days. As Droid Life points it is unclear whether or not this update is only being sent out to those who are actually using the AT&T branded Nexus 6 model or any Nexus 6 which has an AT&T SIM card inserted.
Those attempting to download the update will need to be connected to WiFi according to AT&T, so if you haven't seen the update show up in your status bar as of yet this morning, this may be the reason why in which case you could try connecting to WiFi then manually checking for the software update via the settings/about phone menu. AT&T's changelog states stability improvements and "disable smartlock feature when EAS password policy is enforced" as the changes coming in this update, so users shouldn't be expecting any major tweaks. Of course, if you've been waiting on the update this long, chances are you already have some idea of what the new software includes as you may have likely seen the details pertaining to other models. The update weighs in at about 81MB according to AT&T's details.