So unlike in DVC or HGVC, strictly speaking you can't change a MVC reservation. And you can't cancel a MVC reservation online. Which can create difficulties depending on availability, your points situation, waitlists, and the MVC representative you get either through chat or phone.
It's easy to do if you don't need or want to use the points in the existing reservation and the villa you wish to book is available - you book the new one online and then use chat or a phone call to cancel. But if there isn't availability and/or you need or want to use the points in your existing reservation, you run the risk of some else - in particular some else's waitlisted request - getting the villa on the dates you've given up in your cancellation.