How to Make T-Mobile Work on an Unlocked iPhone
- 1). Press on the hole at the top of the iPhone with a bobby pin to open up the iPhone SIM card slot.
- 2). Remove the AT&T SIM card and replace it with the T-Mobile SIM card you received when you opened an account with T-Mobile. Close the SIM card slot back in.
- 3). Turn on the iPhone. The iPhone automatically configures itself to work with T-Mobile's network.
Source...