Google is currently rolling out the latest and greatest version of Android to the Nexus 9. The OTA will bring your Nexus 9 to Android Lollipop 5.1.1. If you are currently rooted with recovery and are running a custom rom you may have to install the update manually. Normally OTA updates will fail to install if you are running a custom recovery. No worries though developer "Oldblue910" has captured the OTA and has uploaded it. You can simply grab the download place it on your sdcrard and flash it in custom recovery. If you are running custom recovery there is a guide at the link below.
via TalkAndroid