Want to try out Android M? Well, it’s available now! At least the developer preview is. The preview is available for the Nexus 5 (hammerhead), Nexus 6 (shamu), Nexus 9 (volantis), and Nexus Player (fugu).
To download the factory image, hit up the source link at the bottom of the page. It does appear that this developer preview will be updated via OTAs. Of course keep in mind that this is a developer preview and there will be bugs and instability in it. All of the features Google announced today at I/O may not be available at this time.Source: Android Developers