If you have a Samsung Galaxy S III phone then...
Shut off your phone.
Locste the tiny slit on the bottom of your phone.
Take something and gently pry your back cover off.
Remove the battery.
In the area where the battery was, there will be a black flap or piece of plastic. This is where the SIM card is located.
Carefully, slide or pop out your old SIM card and place new SIM card inside.
Make sure when placing new SIM card in that you insert it in just as you removed the old one. Do not try to jam it in or you will damage your SIM card and phone.
If your phone isn't the Samsung Galaxy S III let us know. There are different instructions for every phone.
Good luck!
01-17-2016 12:52 PM