Just call the program coordinator (the secretary). They won't like you if you cancel last minute, because they could have put someone else in that spot, but otherwise it's no big deal. No showing is in very poor taste, and program directors talk to each other, so just use common decency.
totally agree. A nice phone call will solve the situation and gives the program the maximum chance to fill that interview spot. Just as a side note, I have had a couple of friends who, upon calling to cancel their interview due to a time conflict, were offered alternate times that solved their conflict. Wouldn't expect that sort of treatment but it may work to your benefit. A no show is poor form and reflects on you and your school. Don't do it.