This happened to me as well. Simply email firstname.lastname@example.org
and tell them of your situation, including your UC ID, the course you want to drop, and the course you are replacing it with (if any). In a few days, you'll get an email like this
We have received your additional /corrected information and will be forwarded to the campus(es) you have applied to within 5 – 10 business days.
Hope that helps!