The update is only available for download via PC Companion
A Jelly Bean firmware for Sony Xperia J has been certified last month and rumor had it the update will be rolled out in late March.However, it looks like Sony decided to kick off the deployment of Android 4.1.2 Jelly Bean upgrade for Xperia J a few weeks earlier.
According to some XDA Developer forum users, a Jelly Bean update is now available for download via PC Companion desktop software.
Sony Xperia J owner may want to connect their phones to a PC and check for the update. Keep in mind that the upgrade may not be available right away, as Sony usually rolls out these major updates gradually.
The current upgrade will update the firmware to version 11.2.A.0.21 and the kernel has been bumped to 3.4. Stay tuned for more info on this one.