Unfortunately, not all the devices in the Nexus line-up have made the cut for an official Android 6.0 Marshmallow update, but the developer community is already hard at work to bring Marshmallow to the older Nexus 7 (2012) tablet and Galaxy Nexus smartphone.
Starting with the Nexus 7, Dmitry Grinberg, the same developer who recently pushed out a Marshmallow ROM for the Nexus 4, has uploaded a working AOSP ROM, although you’ll have to find a GAPPS package yourself. Everything appears to be working correctly, or at least not major bugs are listed, although the build is for the WiFi only version of the tablet, at least for now.
You can grab the package to make the build yourself, or download a flashable image or zip file from his website via this link.
Galaxy Nexus owners can thank XDA Forum user Ziyan for an experimental Marshmallow build that is currently in development. There appear to be a few bugs here, as the build is in the early stages. Video recordings crash and NFC is not working, and the developer doesn’t recommend it as a daily driver just yet.
If you’re the adventurous type, you can grab the experiment flashable zip file from the download page here. Installation instructions and build notes can be found under the discussions tab.