If you want to update your Nexus 7 to official Android 4.1.2 that started rolling out earlier today but your turn hasn't come yet, you have two options: wait (possibly for a while) or flash it manually. The latter is absolutely safe and lets you bypass the line without any risk whatsoever. Even better - you don't even need to be rooted or running a custom recovery - updating with Jelly Bean and full stock recovery is easier than ever before.
Let's get to it.
Prerequisite: Your Nexus 7 should be running Android 4.1.1 build JRO03D.
Download the JZO54K Android 4.1.2 update zip from Google's servers (32,862,193 bytes, MD5: 1a68f499129ae2ee373806a9cad13174).