You would have to find an IMEI unlock provider online. You will need to know the country of origin of the device, the original carrier of the device, and the IMEI number of the device. Click on the site linked above to ApplenBerry and choose either US or International, where International covers non-US carriers.
Open up the dropdown list of carriers and countries of origin and select the one that matches your device's carrier of origin, and see how much it would cost or whether it's unlockable through ApplenBerry or not. If the price is right, fill in the information accordingly and wait anywhere from 5 minutes to 48 hours for the unlock to take effect.