Steps to unlock Carrier lock

You can't. The cell carrier is the only one who can unlock that phone, and carriers - at least any I've ever worked with or heard of - will only unlock a phone for the person who had the account with the carrier. Return the phone and get your money back.

For future consideration, read this:

The All Too Common SAD Reality of Buying … - Apple Community

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

Step 1......contact the carrier to which the phone is locked

Step 2.....ask the carrier to unlock the phone

But.....the carrier will not do this because you are not the customer that set up service

Step 3.....return the phone for a refund

Or, you could save time and ignore steps 1 and 2 and go directly to step 3