First, switch off your phone. Remove the back of your phone and you should see the battery. The SIM card will either be visible beside the battery, or it will be beneath the battery. Remove the battery to gain access to the SIM card. The SIM card can then be slid out of its holder. You can then replace the SIM with a new SIM. Make sure it is in the correct way and replace the battery and the cover. Switch phone back on.