this is the time for you to act professionally. That being said, you should not just wait until time runs out because you are putting the school in a tough place and you are holding a seat that you do not want from a student who does want it.
that being said. Write a letter or e-mail directed to the director of admissions. Start by telling them how appreciative you are of their offer and how you feel about the offer. Then tell them that unfortunately, you will have to decline their offer. That's it. for example:
Dear Mrs/Mrs./Dr/ Admissions Director,
I would like to start off by tell you the honor that I have in being granted an acceptance to your school. Unfortunately I will have to decline the offer and free up the spot for another student.
Thank you very much for the time you took to consider me. Good luck with your school and I am sure that your incoming class will be very successful.
Sincerely,
M. Edical.
Or you can just say:
Dear Admissions Dude,
Whoa man, I cannot beleive you fell all that crazy $hit that I said on my application and then gave me an interview. I was like, whoa, do thees guys like, like me or what?? I am telling you man, it felt, cool.
then when I went to visit, like, your campus and stuff and that Dr. suess guy, I think that's his name, well that dr guy talked to me, I was like whoa man. I wana be a doctor cause I wanna like make a lotta money so I can like ya know, surf a lot of time man. and he was like, "sir", I never have anyone call me sir before, I was like, yo man....sir is for my father. You can call me dude.
And then he was like "dude, is that your best answer?" and I am like, well, yeah man the pizza guy will be here in like 5 minutes wanna share?
so then when I got your letter telling me that I got in, I was like whoa man that is cool. the I got your like, bill and I was like DDDDUUUUUUUDDDDEEEEE that is a lot, I mean a lot of pot that I would like, have to give up and then that schedule, I was like, whoa are you kidding me? I need that time to surf man.
So admissions, dude, I'm gonna have to say, like no way man. Surf's up