UNLOCK IPHONE TO ANY CARRIER

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

Your first task will be to contact your current carrier to ask them to remove the carrier lock on the phone. Please note that they are not under any obligation to do this. Some carriers will refuse to remove the lock on the phone.

When / if the phone has been unlocked by your current carrier, contact the new carrier to ask them how to set up the phone for their service.