Android 5.1 is rolling out now for most Nexus devices, but as usual it's taking a while to arrive on devices. If you're not keen on waiting around for an update you can take things into your own hands with the official OTA files published by Google. From Phandroid - How to install Android 5.1 on the Nexus 6, Nexus 5, Nexus 7, and Nexus 10