To see if your Sony Xperia M has received the update wirelessly, you need to make sure you have the latest version of Update Centre on your phone. Go to Update Centre > Updates tab and install it. Then, you need to go to Settings > About phone > Software updates > System updates. Sony suggests using a Wi-Fi connection when downloading and installing Android 4.3, so as not to waste your precious data allowance. And you might want to make sure that you have charged your battery to at least 50% before starting this process.
The update includes Android 4.3, which adds graphics and security enhancements and a smoother UI. It also adds a new version of Battery STAMINA mode to keep your battery running longer between charges. The camera on the phone will add Burst Mode, and Sony has updated some apps such as Walkman, Album and Movies.
If you can't get the update wirelessly, you can use a USB cable, hook up to PC Companion, and grab Android 4.3 that way.
Thanks for the tip, Anurag
source: Sony via XperiaGuide