Google has started rolling out a minor software update for Nexus 4, Galaxy Nexus and Nexus 10 devices.Not all the Nexus devices are privileged to avail this update. Many security improvements might be related to a recent Bitcoin wallet case. There is no sign of the update for the Nexus 7 or the new Nexus 7 but it can always appear in the near future according to source.
The Android 4.3 build JWR66Y is truly amazing and the update is around 2MB in size which is not so big compared to other complicated builds. It would automatically appear on your Nexus devices but you can always check updates manually if you are not getting the option by going to Settings — > About Phone — > Update. After the build is installed , you will be able to see that now your device works on Android 4.3.