Google has begun the official rollout of the Android M Developer Preview Build 2 for those of you who have already installed the first dev preview build. If you are not receiving this update yet and would like to go ahead and get it installed the OTA files have been pulled and uploaded! These need to be sideloaded via ADB. The great part about flashing the OTA in recovery is that you won't have to wipe your data. If you install the full image your data will be wiped.
Nexus 5 OTA
Nexus 6 OTA
Nexus 9 OTA
Alternatively you could head straight to Google's download page and grab the appropriate stock image file. Use the Wugfresh Nexus Root Toolkit for a super easy and straight forward way to install your stock image.