Sony have been updating it's unlocked international Xperia Z1S smartphones since the past march to the latest 4.4 KitKat, but users with the phone on contract with T-Mobile was reportedly lagging behind with Android's version 4.3 Jelly Bean.
But fortunately, T-Mobile have finally decided to update the smartphones to 4.4.4 KitKat, better late than never, where the company will be rolling out the software reform notification to users with the device, and then you'll need the Sony's PC Companion software and a computer to get the new firmware installed on your Xperia Z1S, as this is not an over-the-air update. The new upgrade will take you to build number, 14.4.B.0.37 and bringing all kinds of KitKat goodies.
Source: GSMArena