Summarize the following:
Press and hold the Power button on the right side of your phone, tap Power off when it appears, and tap Power off again when prompted. The SIM card slot is typically at the top or on the right side of your Samsung Galaxy's housing, but you may find it inside the battery compartment if you have a Samsung Galaxy S5 or older. Insert the SIM tool into the hole next to the SIM card slot. You can also use a needle or a bent paperclip to open this slot if you don't have a tool. Gently lift the old SIM card out of its tray, making sure not to touch the gold connectors on the card itself. Consider storing the SIM card in a plastic bag or SD container to prevent it from being damaged. The new SIM card should fit into the tray in the same position as the old SIM card. Once the SIM card and tray are properly inserted back into your phone, you can finally proceed with unlocking your phone.

Summary:
Power down your phone. Locate the SIM card slot. Eject the current SIM card. Remove the SIM card from its tray. Place your new SIM card in the tray. Insert the tray back into your phone.