Android "M" preview images for Nexus 5, 6, 9, and Player are now live. Here are the direct download links.
If you run into problems during flashing like the dreaded "missing system.img" error, check out our article with instructions for doing a "dirty" flash (piece by piece) here.
Google has also announced that the "M" preview will be updated more regularly than L's, and specifically that updates will be issued over the air - no need to flash updates yourself. That will be very nice indeed.
- Android "M" Preview