Instructions
1.Obtain an unlock code. T-Mobile users can wait three months from the date their contract starts and then request a free unlock code from T-Mobile. If you're not a T-Mobile customer or wish to unlock your phone immediately, purchase an unlock code online. You'll need your IMEI number, which you can find by going to Settings ->About Phone -> Status.
2. Turn off your Android phone and remove the back cover and battery. Look for the SIM card, and remove the one that's currently in your phone. Set it aside. Insert the new SIM card and replace the battery and back cover. Turn your phone back on.
3. When the phone boots up, it will display a screen asking for an unlock code for the new SIM. Enter the unlock code when prompted. Your phone is now unlocked.
Tips & Warnings
After you unlock the phone, you'll need to set the phone up to use your new carrier's data network.
Unlocking your phone may disable the phone from receiving 3G data, depending on your new carrier. Before you unlock your phone, research to make sure that your new carrier uses the same data band as your old carrier, or your phone may slow considerably.