How expensive would this be?

K so my boyfriend who lives in Denmark bought a plane ticket to come here to California about a week ago. Some things have come up and he's looking into changing his ticket to something earlier like July 20th. He's flying with Scandinavian airlines as well. Can anyone tell me how much it would be for him to change his ticket like this? The ticket hasnt been used yet, obviously, and was purchased fairly recently. Any information would help, thanks a lot.