If you want to do it online, pick your time for Cape May. When you Confirm, if the time is anywhere close to your time for Mama Melrose, it will tell you there is a conflict and ask if you want to cancel Mama Melrose for the new ressie at Cape May. If it isn't close in time and it doesn't create a conflict, go back to the main page of reservations and click "Cancel Reservation" on Cape May and that will take care of it.
If you do it by phone, the CM can take care of both for you. Just depends on if you want to make a phone call or take care of it yourself.
I think I'm going to have to call - when I made our ADR back in February, I used our old email address. Since we changed emails last month, our ADR does not show up online, and they can't be searched. My travel agent called and confirmed them for us though.
DS heard "mussels" and hasn't stopped drooling since!