Did you manage to get into the earliest part of Google's Project Fi carrier program? Good for you. Celebrate your good fortune with the factory image for Android 5.1.1 build LVY48C on the Nexus 6, which went out in over-the-air form last week. There aren't any major user-facing changes between this version and other N6 builds, just some under-the-hood changes to make Google's unique combination data system work.
Remember, OTA files are for updating stock software, and factory images are for returning to stock software. If you've modified your Nexus 6 with root or any other major changes, you won't be able to flash the over-the-air file. Download this file and flash it via Fastboot from your PC (and say goodbye to your apps and data) to get back to the stock update branch.