It has been nearly three months since the announcement of Google's latest Android OS, Android M. While we now know that it will be officially dubbed 'Marshmallow', we most likely will not see it in an official capacity until Google announces their latest Nexus devices later this year. For now though, Google has unveiled the Android 6.0 SDK & the third and final developer preview for Android M.
For those that are interested in developing apps or revamping their apps for Android Marshmallow, there is a new Android 6.0 SDK available that can be downloaded via Android Studio. Google has also released the third and final developer preview for Android M. Currently, the Android Marshmallow developer preview is available for the Nexus 5, Nexus 6, Nexus 9 & Nexus Player.
For those interested, you can download the appropriate system image for your device from the download page. Remember, by downloading and tinkering with your device, you are putting your device at risk. If you are not familiar with the process or how to fix it if something goes wrong, it is highly recommended to wait for an official release or update.
Source: Android Developer Blog