How to Switch SIM Cards on an iPhoneby Michelle Varsallona
The SIM card on your phone holds all the transferable data. This includes messages, contacts and calendars. You may find yourself needing to replace your iPhone's SIM card for several reasons. It may be damaged and crashing your phone, or you may want to import one from a different phone. Most SIM cards on cell phones are located underneath the phone's battery and are tough to reach, but the iPhone simplifies the process.
Locate the SIM card tray. On 3G iPhones it is at the bottom of the phone and on 4G phones it is on the right side.
Use the paperclip to press in the small dot on the tray. This causes the tray with the SIM card to pop out.
Remove the SIM card from the tray and place the new one inside.
Insert the tray back into the phone and press until you hear it click.