Back in May of this year Sony pushed Android KitKat 4.4.2 update to its original Xperia Z lineup. Now the OEM is further lifting up these devices to the latest Android KitKat version 4.4.4. With today’s update Xperia Z, ZR, ZL and Tablet Z will be put on the same level as Sony’s flagship devices, in terms of software version. Sony is definitely serious about maintaining its devices. The latest update brings bug fixes, improvements to battery and WiFi performance and enhances email experience on these devices.
— Sony Xperia News (@SonyMobileNews) September 9, 2014
According to XperiaBlog the build number for this update is 10.5.1.A.0.283. Since this is an OTA update, your handset or tablet should notify once its available for your region and/or carrier.