After the Xperia Tablet Z LTE owners were surprised with the Android 4.3 jelly bean update its now turn for the WiFi variant owners of the Tablet Z.
The new update is live for the SGP311 and SGP312 Wi-Fi models, according to Xperia Blog, and will bring your firmware up to version 10.4.B....
Xperia Tablet Z Android 4.3 Jelly Bean Update Comes for WiFi Version