First check actual software version (it can be found in the menu of the phone) then take a look here:

Downloads - W580 - Top Sony Forums

and locate your serial then enter and search for your branded firmware. Next you can download files wich are needed four a backup in case you need warranty return (thats MAIN, FS and CDA), so in case you'll need to return the phone, flash it with those firmware.

Then locate latest firmware and debrand it.

You can then play with the software, BUT remember to back up GDFS first time because it's unique to every phone.
If you want to pay for unlock then contact one dedicated server.

Here someone more experienced could tell if in case of returning it is posible to lock the phone again so the store never tell if it was unlocked.:)