Android 5.1 Lollipop is still making its rounds on the different Nexus devices that will be supported for the version of software, and while not all the devices yet have it, Google has just recently posted the Android 5.1 Lollipop (LMY47D) factory images for the 2012 Nexus 7 mobile version. The 2012 mobile Nexus 7 joins the Nexus 6, Nexus 5, Nexus Player, Nexus 10, and 2012 Nexus 7 WiFi version in Android 5.1 officially, and if you have been waiting to update your Nexus 7 2012 mobile to Lollipop and don't want to wait for the OTA to hit your device, you can grab the download of the software below and flash the factory image manually for your device if you are familiar with the process.
If you have never flashed a factory image manually, you can follow the steps in this post here to complete the setup. Owners of the 2013 Nexus 7 in both the WiFi and LTE models will still have to wait for the factory images of Android 5.1, but with Google taking care of the 2012 mobile model today perhaps those with the 2013 LTE Nexus 7 will get their dose of factory images sometime in the near future. It's possible that they could end up showing up throughout the day, and if they do appear on the factory image webpage we'l be sure to update.
Most users should be familiar with what comes along in the Android 5.1 Lollipop update. While most of it is bug fixes and improvements, there are some changes to the software and user experience and a few new features for compatible devices. HD voice for example is a feature that Nexus 6 owners have access to with the 5.1 Lollipop upgrade. Unfortunately that isn't something that's compatible with the 2012 Nexus 7 mobile model. Other changes include the dual-sim support for compatible devices, most likely only the Android One phones at this point, as well as Device Protection. Again, these are factory image downloads and you should only flash the software manually if you feel comfortable with the steps included in the process, otherwise it might be a safer bet to just wait for the over-the-air update to come to your device.