Power off the iPhone
Locate the SIM tray on the side of the iPhone
Insert a SIM eject tool or a paperclip into the small hole next to the tray
Press gently until the tray pops out
Remove the SIM tray
Take out the old SIM card
Place the new SIM card into the tray in the correct orientation
Reinsert the tray into the iPhone
Power on the iPhone
Check for cellular signal and carrier connection