Sony Mobile is updating as we speak the Xperia TX LT29i to Android 4.1.2 operating system. The update should be available in some areas (Spain for sure) via Sony PC Companion tool, others who are more experience went ahead and flashed the new firmware build 9.1.B.0.411.
Our advise is to try plug-in your Xperia TX to your PC/laptop via USB cable and check using Sony PC Companion software tool if it is available at your location. Those that know how to flash a ROM you can go ahead and download build 9.1.B.0.411 from the following mirros below:
If you’re looking for a how-to tutorial head over to XDA forums here.