Turn off your iPhone
Locate the SIM tray on the side of the iPhone
Insert a SIM eject tool or a paperclip into the tray hole
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 position
Reinsert the SIM tray into the iPhone
Turn on your iPhone
Wait for the iPhone to detect the SIM card
Check for signal and carrier name
Restart the iPhone if the SIM is not recognized