How do I remove the SIM lock from my iPhone?

Contact the carrier your phone is locked to, they are the only ones who can unlock it.

Note: If you bought the phone from someone and it was locked, the carrier is under no obligation to unlock it. Usually they will only unlock it if your are a customer in good standing with them.

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

If it’s thru your network then you will have to pay for your phone or have your service thru the services contracted time limit. If you signed a two year contract then it will be two years