Manually Update Nexus 7 To Official Android 4.1.2 (JZO54K) Jelly Bean

*Nexus 7 must be rooted.
 
Step 1: Download Android 4.1.2 Jelly Bean (JZO54K) [Official Google]
Step 2: Place it directly under /sdcard on your Nexus 7’s storage.
Step 3: Reboot to ClockworkMod (CWM) Recovery.
Step 4: Navigate to install zip from sdcard > choose zip from sdcard and then select the file you transferred to your Nexus 7 in Step 2.
Step 5: Let CWM do its thing.
Step 6: Select reboot system now from the main menu.
If you followed the steps correctly, your Nexus 7 will now boot into Android 4.1.2 Jelly Bean (JZO54K). You can confirm this by going to Settings > About.