You can change your dates at any time if it is a room only. BE SURE to make changes as quickly as you can to avoid any "upcharging"
If you are on a package, you have until 45 days before your check in date to make changes. After 45 days (not sure exactly the cut offs after that) they can charge you to make changes (although I've found they don't if they can help it at all)
Please let us know if it is a room only or a package, that will make it easier to troubleshoot. have fun!!