For many Nexus owners, the wait for Android 5.1.1 is finally over. Today, OTAs began rolling out for several Nexus devices which had thus far been stuck on Android 5.1. If waiting on an OTA update isn't your thing, you can now head on over to Google's Nexus developer page to get the latest factory images.
In order to flash these, you'll have to have some familiarity with unlocking bootloaders and using Google's Fastboot utility. Plenty of easy tutorials can be found with a simple Google search.
With these latest updates, the only Nexus that still doesn't have an official Android 5.1.1 update is the Nexus 6, but considering the latest push to AOSP was completely for that device, an official OTA likely isn't far off.
- Google Developers