You can make this change in your Orlando Informer account. Simply sign up or sign in here, then find the "Modify Reservation" button under your hotel stay.
Choose to change dates, but select the same dates as your original reservation. You'll be able to rebook at the current cost, pending room availability. If the new rate is lower and you've already paid in full, you'll receive a refund for the difference.
Please keep in mind that hotel rates are variable and may also increase, so check the new rate carefully. Additionally, due to Universal Orlando restrictions, all modifications are processed by cancelling the previous booking and creating a new reservation. As a result, cancellation fees may apply to the original reservation if your check-in date is less than 6 days away.