Android M developer preview and SDK now available for download
Take heed that the developer preview version is in no way a final product – a lot of the new features may still be underdeveloped or still not implemented. The flashing process will wipe your phone, so you will need to backup everything important. It is not a complicated task and Google has posted a comprehensive guide on how to do it (linked below), but we'd still advise against it if you don't feel confident in your tech savviness. The good news is that once the preview Android M has been flashed on your device, future updates will come over the air. You will not need to repeat the tedious process after each little update – and a number of those are bound to come over the course of the next few months.
The developer preview of Android M can be reversed back to Android Lollipop by flashing back a build of your choice (downloaded from the Android Developers website). Without further ado, here are the links: