You have to know the exact model of your dongle and its IMEI, then you can use various tools (eg. Universal MasterCode) to unlock the dongle. After unlocking, another tool needs to be used to flash the firmware that is also based on the model number. Since this is a rather complicated process, visit a local phone service shop based in your area and I believe they will flash it for you for a very small fee.
Please keep in mind that if the dongle appears to be under contract, its warranty will be void.
