If you are able to retrieve your dining reservations online you are able to go in through there and cancel. However, I recommend that you rather phone them. My reason for saying this is that there have been reports, albeit randomly, that people with double reservations had both "auto cancelled" by the system - it seems to be a bid on Disney's behalf to clamp down on folk double booking, not like you, but deliberately and then only pitching up for the one they want. As I say, reports are random, but enough of late to take note of and I would personally rather speak to someone and be assured that the one I wanted to keep is still on the system. I am really sorry if this causes you any unnecessary concern, but I am just putting myself into your situation. If you need cheap call numbers for the US (1p per minute), let me know.

Their phone lines close at 1 am GMT (9 pm EST)