Tips
1 – Power off your Iphone because it is necessary for you.
2 – Keep in mind, you sim card must be accurate size for your iphone so when you buy simcard tell your buyer about your iphone model.
3 – See where is the simcard interning option so you must know about it.
4 – There may be two option in your iphone such as SIM eject tool or straighten a small paperclip so one option from both, will be in your iphone.
5 – You have to put SIM eject tool or straighten a small paperclip into your SIM hole.
6 – Out Your SIM from iphone easily.
7 – Add new SIM card into your iphone after removing old one.
8 – Now you have to tray back into your iphone.
9 – Turn on your iphone now your new SIM card will be connect to your new network easily.
Note – if you still not understand above tips, you must follow wikihow which is very well written with images for you.