Google's Android team announced on Twitter at 12 Nov, AndroidLollipop rollout has started and will soon be available on most Nexus devices. Nexus 4, Nexus 5, Nexus 7 (both models), and Nexus 10 will all start to receive Lollipop over-the-air (OTA) shortly.
If you can't wait for the OTA, you can download it from the page of Factory Images for Nexus Devices. This page contains binary image files that allow you to restore your Nexus device's original factory firmware. You will find these files useful if you have used the Android Open-Source Project, flashed custom builds on your device, and wish to return that device to its factory state.