Sony Mobile has just confirmed via Twitter that a maintenance software that will bring Android 4.2 firmware update, is already underway in phases for the following devices: Sony Xperia Z, Xperia ZL, Xperia ZR and Xperia Tablet Z.
According to @SonyMobileNews the new firmware build is ment to bring display, camera, battery fixes and performance improvements to the aforementioned devices.
Back in July, Sony detailed Android 4.2 and Android 4.3 upgrade rollout for its Xperia devices, and started pushing out firmware updates to Xperia Tablet Z 4G LTE model in early August, and more recently to Xperia ZR.
The update should be available through an OTA (over the air) notification or via Sony PC Companion tool. We are not aware in which regions the update is available, however you can try searching on your device by going into the Settings > About device > Software updates > Check.
You can also go ahead and download the latest firmware build 10.3.1.A.2.67 for Xperia™ Z, Xperia™ ZL, Xperia™ Tablet Z, Xperia™ ZR from Sony Developer website here for a manual install, that if you know what your doing.