The first update to the Android M Developer Preview has just been released, bringing with it a number of changes. Back at Google I/O, Google released the first build, with the expectation of releasing a new build each month until release. Preview 2 includes the up-to-date M release platform code, some almost final APIs for testing, and a number of other changes.
If you are currently running the Android M Developer Preview on your device, you can install the update over-the-air. For those who are not currently running it, or prefer to load it from a computer, Google has posted the new system image as well. You'll need a Nexus 6, 5, 9 or Player to use it.
Source: Android Developers