How to fix Nexus 7 app update errors after installing Android 4.2
Overall, the Android 4.2 update seems to be going pretty well and hasn't had any major bugs that have been reported yet, but Nexus 7 owners have been reporting a minor, but annoying error after installing the update. Apparently, some users have found that after installing the Android 4.2 update, the Google Play Store won't be able to update apps and will keep giving an error, which reads that the update couldn't be downloaded because of an error.
Luckily, there is an easy fix if you've come across this problem. It seems that it's a problem with the Google Account information, so all you have to do is go into Settings>Accounts, delete your Google account, then reboot your device, and re-enter your Google account info. Simple as that.
It should only take a couple minutes, and then you'll be right back to updating apps without any issue.
source: Droid Life