With the release of Android 4.2 yesterday your excitement is probably growing by the minute in anticipation of the new update on your device. Google has already begun the process of updating the GSM versions of the Galaxy Nexus and the Nexus 7, but sometimes the OTA rollout can be a bit slow. If you are chomping at the bits to try 4.2 no worries. There are a few methods of manually updating.
Method One is for those of you who are completely stock. You have never unlocked your bootloader, rooted, or installed any sort of custom recovery. If you look in settings under about tablet you should be on Android version 4.1.2 and build JZO54K. You will also have to have the SDK installed on the PC.
If this is you proceed to manually update.
Step 1.) Download the update.zip from AndroidPolice
Step 2.) Power down the Nexus 7 and boot into stock recovery (hold power+volumedown, scroll to recovery using volume down, select with power) when the android guy pops up press power+volumeup to bring up the menu
Step 3.) Connect your Nexus 7 to the PC, Navigate to the folder where the file above was downloaded, open a command prompt and type "adb sideload 094f6629314a.signed-nakasi-JOP40C-from-JZO54K.094f6629.zip"
Step 4.)Reboot and Enjoy!
Method 2 is much easier but requires that you already have an unlocked bootloader, be rooted, and have a custom recovery installed.
Step 1.)Download the Stock Rooted JB 4.2 JOP40C rom via XDA directly to your device.
Step 2.)Reboot to custom Recovery
Step 3.) Wipe Data, Wipe Cache, Wipe Dalvik Cache, Install zip
Step 4.) Reboot and Enjoy!
Personally I would just go with Method 2, but that is just me!