Despite exclusion from the official Marshmallow list, Google Nexus 7 (2012) can now be updated to Android 6.0.1 Marshmallow using a custom ROM. The device is now eligible for unofficial Android 6.0.1 Marshmallow firmware via the Android Open Kang Project (AOKP).

Google Nexus (2012) is luckily one of the few devices to receive Android 6.0.1 Marshmallow that is packed with better battery performance, aftermarket features, and enhancements that fare well with the official build. However, AOKP is still in beta stage which means bugs can be expected to appear.

Before proceeding with the installation, here's a bunch of reminders and prerequisites to consider:

1.     Never try the ROM on other versions or models not prescribed.
2.     Download and install USB drivers for this version.
3.     Enable USB debugging mode from developer menu.
4.     Download and install a custom recovery tool on the phone like CWM or TWRP.
5.     Back up necessary data.
 
To download and install AOKP ROM to Google Nexus 7 (2012), here's how:
 
1.     Download the AOKP Android 6.0.1 Marshmallow ROM here.
2.     Download the Google Apps here.
3.     Connect Nexus 7 (2012) on PC using USB cable then copy downloaded files to phone.

4.     Press hold simultaneously the volume +, home and power buttons to restart phone into Recovery mode.
5.     Perform Nandroid backup of the existing ROM. This will allow users to get back to the original software version once ROM is updated. To do this, go to Backup and Restore > Backup again. Once back is complete, go back to main recovery menu.
6.     Choose "Wipe data/factory reset". Tap "Yes" to confirm action.
7.     Once completed, go back to CWM recovery menu > Wipe Cache Partition > Wipe Dalvik Cache.
8.     Tap Select Install zip from sdcard > Choose a zip from SD card. Find the downloaded AICP ROM and select it by pressing the power button. Tap "Yes" to confirm action. Installation shall begin.
9.     Once completed, go back to third step to install the Google Apps.
10.     Once completed, go back to the main recovery menu. Select "Reboot system".
 
Once completed, the tablet should now be in Android 6.0.1 Marshmallow. To check the software, go to Settings > About device.