Turn off the phone.
Eject the Sim tray.
Remove existing MicroSIM from tray.
Place new MicroSIM in tray.
Re-insert SIM tray.
Turn on phone.
Note: If your phone is locked to AT&T, You can NOT use a foreign SIM card.
If your phone is from Verizon or Sprint, you need to have them unlock the GSM side for International roaming BEFORE you do this.