YOUR DRM KEY WILL BE LOST, SO BACK UP YOUR DRM KEY WITH BACKUP TA PARTITION, HERE THE LINK http://forum.xda-developers.com/show....php?t=2292598
AND YOUR WARRANTY WILL BE LOST, DO WITH YOUR OWN RISK!
SO, IT'S UP TO YOU!
OKAY, HERE WE GO...
1. In your Xperia M, and dial service center *#*#7378423#*#* select Service Info -> Configuration -> Rooting Status, if Bootloader unlock allowed says Yes, then you can continue with the next step. If it says No, or if the status is missing, your device cannot be unlocked.
2. Open this link http://unlockbootloader.sonymobile.com/unlock/step1 and select "Yes, I'm Sure", Tick all the terms, Select "I'm Accept", Enter your name, a valid email adress and the first 14 digits of your device’s IMEI number , Click Submit. (The unlock bootloader key will be sent to your email, save it)
3. Shut down your Xperia M.
4. Open Flashtool (You can download it from here http://www.flashtool.net/index.php )
5. Select BLU button, beside lightning icon.
6. After an image pop up, connect your device with holding volume down/ decrease ( - ) button.
7. Enter your Unlock Bootloader Key (Step 2) and select unlock.
8. After finished, close the flashtool application, and disconnect your device.
9. Turn on your device and dial service center (Step 1) if you're succesfully, it would be like this:
DO WITH YOUR OWN RISK
Sorry, if my english is bad