If you are the owner of a 2012 Nexus 7 3G (Nakasig) then you are in for a treat today, as Google has just posted the factory image that will update your tablet to Android 5.1. There’s no news on when the update will be available OTA as yet.
The Android 5.1 update comes in at 403MB and is available to download from Google’s Android Developers website linked below. Once downloaded, just follow the steps as posted in this guide to flashing and you will be on your way to enjoying a second helping of that lollipop goodness on your 2012 Nexus 7 3G tablet. The Android 5.1 update includes new features that includes Device Protection and the usual bug fixes and performance improvements, more of which can be read about here while you wait for the flashing process to complete.
Source: Google Developers Website