How to unlock iPhone to use SIM card

You will have to contact you current carrier about that, check this article:

How to unlock your iPhone for use with a different carrier - Apple Support

The contact info for your current carrier should be listed on this page:

Wireless carrier support and features for iPhone in the United States and Canada - Apple Support

Just to be clear, “current carrier” refers to the phone’s original “subsidizing” carrier — i.e. the carrier who applied the “carrier lock” to your device.

If you are no longer an active/paying customer of THAT carrier, you might find that “device unlock” requests are a “challenging” task.