Hi Debora,
Welcome to planDisney! I am so excited to know you are planning a visit to the Walt Disney World Resort! 
First things first, let's make sure your reservation is linked in 
My Disney Experience. If you do not have a My Disney Experience account, you should create one. Then, use the confirmation number to link the reservation in My Disney Experience. Once it is linked, log into My Disney Experience and check My Plans to see if you see a link to Change Reservation. If you do not see a link and/or cannot modify your dates online, you can certainly 
contact a Cast Member by phone.
As long as those dates are available, you should be able to add them. If they are not, you may have a dilemma. If the additional dates are unavailable, check the pricing and availability for other resort hotels. Also check to see if there are any new 
Special Offers now available that were not available when you first made the reservation.
If you have not yet purchased theme park tickets, I suggest that you do so immediately. You need theme park tickets linked in My Disney Experience to make 
Disney Park Pass system reservations. Each Guest currently needs a park pass reservation to enter a theme park. Check the 
theme park reservation availability calendar. When I just checked, there was park pass availability for your dates.
As you continue planning, I hope you have considered making 
advanced dining reservations. In the summer heat, we appreciate dining reservations for the air conditioned break. A few of our favorite restaurants include 
'Ohana, 
California Grill, 
Be Our Guest Restaurant, and 
Sanaa.
Debora, I am sending some pixie dust your way trusting that you get that reservation modified! We cannot wait to welcome you to a world of magic!
Magic and memories,
Donyell