how do I turn down this internship for another?

I have a phone interview today with CNN internship. I want the position. I also have an interview in person, tommorrow at Time, Inc. for an internship. If I'm offered the internship today, I'm going to take it - question is: what do I do about my interview scheduled for tomorrow? I don't want that position if I get this one.should I call and cancel? email and cancel? go anyway? If I go anyway and they offer it to me - what should I say (knowing I don't want it anymore). Help!