At this year Google I/O, Google promised monthly updates to Android M, so the Developer Preview of the latest version of Android has received its first update.
This Preview 2 includes the new platform code, some additional APIs for testing as well as a few developer reported fixes such as Camera Permission and TextInputLayout issues.
If you are already running the M developer preview on a supported Nexus device like, Nexus 5, Nexus 6, Nexus 9, or Nexus Player, the update should arrive via OTA. It is expected that all devices currently on the developer preview will receive the update over the next few days.
There is a new version of the Android M Developer Preview here for any new installs for devices.
Google says that the official release of this new Android version is still schedule for fall and asks for anyone on the Developer Preview to report critical issues as you continue to test.