From the fine folks at Android Central, I give you the definitive guide to getting Android 4.1.2, Jelly Bean, on your Nexus 7 device:
Caution: This guide is intended for technically proficient users only. Proceed at your own risk. Dragons ahead, etc.
- A completely stock Nexus 7 running Android 4.1.1, build JRO03D. (Check under Settings > About device; if you have a different build number, you may need a different update package.)
- Our zip file containing ADB executables for Windows, Mac and Linux.
- The Android 4.1.2 update package from Google. (31MB)
Check past the break to see the full manual update process.
- Unzip the zip file containing the ADB executables into a folder on your computer.
- Place the update .zip package from Google in the same folder, but do not unzip it.
- On your Nexus 7, go to Settings > Development options and switch the toggle on. Then enable USB debugging under the “Debugging” menu group.
- Power down your Nexus 7
- Start up the bootloader by holding down volume up, volume down and power on your tablet.
- When the menu loads, plug your Nexus 7 into your computer.
- Use the volume up/down keys to navigate to “Recovery mode”, then press the power button.
- When the Android with the red exclamation point appears, hold down power first, followed quickly by volume down and volume up. (So that to start, you’re holding down power, then you’re holding down all three). When the menu appears, release all three keys.
- Use the volume up/down keys to select “apply update from ADB,” then press power to select it.
- On your computer, open up a command prompt or terminal window.
- Using the command line, navigate to the folder containing the ADB executables and the update file. (cd command)
- On Windows, type:
adb.exe sideload 03a4eaf95f73.signed-nakasi-JZO54K-from-JRO03D.03a4eaf9.zip
… or on Mac, type:
./adb-mac sideload 03a4eaf95f73.signed-nakasi-JZO54K-from-JRO03D.03a4eaf9.zip
… or on Linux, type:
./adb sideload 03a4eaf95f73.signed-nakasi-JZO54K-from-JRO03D.03a4eaf9.zip
- The update should begin installing. When it’s done, select “reboot system now.”
- Enjoy Android 4.1.2 on your Nexus 7!