Because of the radically different design of the iPhone, you may believe that it is not possible to change the phone's SIM card at home, or not be sure where the SIM card on your iPhone is located. It is possible to change the SIM in an iPhone, however, and it is no more difficult than changing the SIM card in any other model of mobile phone. Changing an iPhone's SIM card only takes a few minutes and requires no tools except for a pin or other small piece of wire.
Look at the top of the iPhone and locate the SIM card slot between the power button and the audio jack. The slot will be covered by a small panel with a round hole in one end.
Insert a pin or wire into the hole, pressing down in order to push the panel slightly into the iPhone. Push until you hear a click, after which the panel and its attached SIM card tray will pop up and stick out of the phone slightly.
Remove the SIM card tray from the iPhone and slide the card out of its mounting brackets.
Insert the new SIM card into the tray, making sure that it is oriented in the same position as the original SIM card.
Slide the tray back into the iPhone, pushing it in until it clicks into place. You may have to use your wire to push the tray in completely.
Be aware that user maintenance may void the warranty on your iPhone.