After the Update was launched, last week we published a list of devices that will get the new Android 6.0, some would be via OTA, and others will have to be done manually, precisely via Flashing it into the device.
Google have released some Download Links for the Android 6.0 OS for manual installation for some NEXUS devices. These includes:
Nexus 7 (2013)
Nexus 7 (2013)
You can upgrade or Install Marshmallow 6.0 manually on any of those Nexus devices listed up there.
Download links for Nexus Android Marshmallow 6.0 OS
Go to Google Developers and download the Marshmallow binary files for your Nexus version or model.
How to Pick the right Android OS or Marshmallow for your Nexus.
1. On the page, Locate your device type, it could be Nexus Player, Nexus 9 (LTE), Nexus 6 etc
2. After finding your device model, in the section where you see "Version" scroll down to see if you find 6.0 there, that's the Marshmallow. Click the "Link" on the page to Download.
Flashing Android Marshmallow 6.0, into your Nexus device
There are instructions on the Page for you to follow, this will help you select the right binary image files for your Nexus and flash them into the device.
As reported by editweaks For More Detailed explanation on How to manually install it on a Nexus device (using Windows and Linux operating system), check out this full tutorial on Android Authority.
The tutorial there was for Lollipop upgrade, but it also works at installing Marshmallow in all Nexus devices.
Photo credits: android authority