If I remember correctly the SIM tray is on the left side of the phone near to the top. You will see a small hole by the side of the tray in which you need to push the SIM realease tool. If you don't have one unbend a small paper clip and insert the end into the hole. This will release the SIM tray so you can replace the SIM card.
If the SIM card tray is not where I said it will be somewhere on the side of the phone.