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http://www.sonymobile.com/global-en/software/phones/xperia-m/
You may already have the latest software, depending on the date of purchase. Software updates are released at different times depending on operator and/or country. Therefore, we cannot give you an exact date for when a specific update will be available for your device.
Android version: Google Android 4.1 (Jelly Bean)
Latest available software: 15.1.C.2.8
Release started: 2013-11-13
if u not recive a notification it's not mean your phone is can't update
you can update from sus
http://www.sonymobile.com/global-en/tools/update-service/
follow the instruction and you can update to the latest software
good luck