With Android Marshmallow expected to be released Over-the-Air (OTA) tomorrow, Google is already paving the way for that to happen. They have just posted the Factory Images of Android Marshmallow for the Nexus 5, Nexus 6, Nexus 7 (2013), Nexus 9 and Nexus Player on the Android developer website for those of you who have been running the preview of Marshmallow or want to root your device and install it today instead of waiting for the OTA.
Unless you know what you are doing, I strongly advise waiting to get the OTA instead of doing the Factory Image install but the instructions for doing it are on the site.
For each of the devices the new build is Android Marshmallow is 6.0.0 and build MRA58K. Keep in mind that if you are upgrading your Nexus 7 (2013) you will need to make sure you get the
image for the Wi-Fi only or Mobile version. Interestingly there is not a seperate build for the Nexus 6 users on Project Fi as there was in Lollipop. This seems to point to Google having one build for the Nexus 6 and it sorting out what service you are using in the install process. Note that there is no image for the new Nexus 5X and Nexus 6P yet as those devices, when they do ship, will have Android Marshmallow on them. I expect we will see future images for these devices as 6.x.x is developed.