It works for both single and dual sim versions (tested)
First: Downloads
Second: Steps
1- Install Sony flashtool
2- Go to C:/flashtool/drivers open flashtool-drivers.exe
3- Select Xperia M drivers on installation, if you don't find it, install both flashmode and fastboot drivers
4- Copy C2005_15.2.A.1.12_CE.ftf to C:/flashtool/firmwares/
5- Open Flashtool program itself, click on flash, choose the downloaded firmware then click ok
*If you don't want to delete user data on your phone uncheck userdata.sin first
6- Start flashing process, when Flashtool shows pop up window, connect phone to PC while holding volume down button
7- Wait few minutes, and you're done
