Remember Google I/O 2015 that occurred a few months ago and the Android M Developer Preview has been announced at that event, after the official unveiling of the most important new features in the latest OS release. During the event, Google promised that the Android M Developer Preview would receive a couple of updates from time to time before the OS becomes stable enough to be officially released to the consumer.
The schedule of releasing the Android M Developer Preview is slightly late, but the first update for the Android M Developer Preview is already out and you can grab one of the new system images from the developer preview website and manually install. Besides that, you can wait for your device when the provider is ready to push out and it will notify you instantly.
At the first in the history for Google to release the previews with over-the-air updates enabled. The company estimates that all the devices have the original preview image installed will be receiving their update within a few days.
All these previews will be only available on the LG Nexus 5, Motorola Nexus 6, HTC Nexus 9, and ASUS Nexus Player. The update contains fixes based on developer feedback and it comes with the up-to-date M release platform code, as well as near-final APIs.
Fortunately, the final release of Android M will become available sometime this fall if they follow the initial planned.