usally the sim card is seated under the battery, power off the phones, take off the both back covers, remove the batteries and slide the sim cards out and switch (depending on the nokia phone the sim card may be in a plastic holder or grove) note: if you changed service providers your phone may be locked and your old sim card wont work with it, or if you went from a gsm provider ro cdma or vica versa one of your phones wont have a sim card.