Every iPhone has a SIM card slot on his right side. This slot contains a tray containing the SIM card of your iPhone. This SIM card allows your phone to connect to your carrier so you can create and obtain mobile data.
You may never need to access your SIM card if you purchased your iPhone with a pre-installed device from your service provider. However, if you buy an unlocked phone and no SIM card, or if you already buy used, you will need to become familiar with what a SIM card is and how to exchange it.
Fortunately, accessing the SIM card tray of an iPhone is not complicated, but you will need three things before you start.
An iPhone (obviously)
The SIM card you want to install
A tool to pierce the side of your iPhone and eject the SIM card tray
The latter can be a little tricky. Depending on your iPhone, a SIM card removal tool can be provided. If that's the case, you're ready to go. If not, they are stupid-cheap on Amazon, but you can also use an unfolded paper clip, a needle or almost anything that is thin and pointed. Just try not to stab yourself as you go.
Once you have your SIM card removal tool (or proxy), insert it into the small hole that is part of the SIM card tray. You should feel some resistance and you have to tackle it a little bit. If this is the first time you do this, it may sound a little strange, but you need to exert a little bit of strength to pull out the bin. Once you've done that, the board will start to eject, and all you need at that point is to pull it out.
Once the tray is out, remove any existing SIM card and install the new one. Make sure you have common sense using the notched corners as a guide. Once the card is installed in the drawer, reinsert it into your iPhone, making sure that the pinhole is aligned with the one on the phone.
Once the card installation is complete, your iPhone should recognize it without any other device rebooting. Otherwise, try turning on and off the Airplane mode or restarting the iPhone completely. This should allow you to work well.