Call the admissions office of the school you want first. Make sure they know you are withdrawing from the other school otherwise they could drop you.
Once you are sure they are not going to take back your acceptance, call the school you want to withdraw from. Talk to someone on the admissions committee, then ask if they want you to follow up with a letter, email, etc.
Make sure to call the school you want to go to! AMCAS has sent out the data and I know my school wasn't too psyched to see that I had two spots (even though I withdrew two weeks ago).
And you might as well do it right now. The decision won't be any easier in one hour, five hours, or tomorrow when it's too late.