Alternative Android OS developer Cyanogen has announced that Android 6.0 Marshmallow-based CM13 builds are now available for download. However, as of now, only select phones have been enabled to receive the nightlies.
The device list currently includes OnePlus One (bacon), Nexus 7 (deb & flo), LG G4 (h815), Galaxy Tab Pro 8.4 (mondrianwifi), LG GPad 7.0 (v400 & v410), LG G3 Verizon (vs985), and Moto X 2014 (victara). The company says that more devices will be added to the list soon.
Cyanogen also wants you to be aware of a few details on how the new builds will impact your experience before you go ahead and flash them. The company recommends that those currently on CM 12.1 YOG4P or CM 12.1 YOG7D releases should not jump to nightlies unless they are "willing to accept a dip in quality." Quality approved snapshot for CM13 is expected to arrive sometime in January 2016.
If you are on a 12.1 nightly, and want to flash the new build without using the "Wipe Data/Factory Reset" option, you'll also have to update your third party addons in the same pass. "This means, if you use Xposed, OpenGapps, etc. flash a Marshmallow compatible version of them immediately after your CM13 flash," the company said in a blog post.
Finally, keep in mind that the upgrade is not compatible with any third party distributions. So in case you're running one, you should do a full wipe before making a switch.