Assuming you are talking about when sailing on
DVC points - when you arrive at the Port to check in, just request to see the supervisor regarding upgrades. If there are none, there is usually a sign posted indicating that. If they are available, you will be directed to a desk where they will show you the available category and/or cabin and the price (in cash) charged to upgrade.
We did this on our Baltic Cruise last summer and upgraded from an inside cabin to a Navigator's Verandah. We had spent all our points to make the reservation and just hoped we might be able to upgrade one of our staterooms. Lo and behold, we were lucky!
Do get to the Port early if you are hoping to do this though..