I actually just had to do this, but with two different schools. I had one on Jan 31st, while the other was for Feb 1st. I had already accepted the Jan 31st one by the time the other came around, so I sent am email stating that I had already accepted another interview but was still very interested and after researching flights I would be unable to make the day/time they had for me. They got back to me quickly asking if the later time would work and if not I could do a phone interview. The main thing is to stay professional while being direct. Hope this helps if you have any other questions feel free to PM me.