Just like canceling a reservation for an excursion, I would think, go to your reservation, to the place where your itinerary is. Click on the "day" where the reserviation you want to change is. There is a "modify" link.
I don't know for sure, but my guess is that it is like any other reservation--you can change/cancel up to 3 days before embarkation on line. Don't know what the on board policy will be.
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