Android 6.0 Factory Images available for Nexus
Google is currently rolling out the latest Android 6.0 “Marshmallow” to their Nexus devices. Although, Over-The-Air update might take a while before it reaches your region. So, what do to do if you want to have it right away? Flash it!
Flashing the Android images needs to be done properly, please proceed with caution. Remember, you can Google for help whenever trouble arises.
Head over this link to check the list of available factory images for various Nexus devices.