Sony has released Android 4.3 update for Xperia M Dual. It will be available as an OTA update worldwide soon. If you cant wait, download Flashtool and the firmware file (ftf) from the links below and flash away. Flash at your own risk and dont ask for help here. Use XDA forum for your queries.
Flashtool: http://www.flashtool.net/index.php
Android 4.3 Update: http://naruto.jubair.info/Xperia_M_Dual_15.5.A.0.18_C2005.ftf
