Hello, Theresa! I'm so happy to be able yo answer your very first question here at the Disney Parks Moms Panel...I hope it's not your last!
If you booked your Walt Disney World Resort reservation directly through Disney (either online or over the phone), and have it linked to your
My Disney Experience account, you may be able to make changes online on the "My Plans" page. If your reservation is eligible to be modified, you'll see a link just beneath the confirmation number that says "Change Reservation." Once you select this option, you'll see a series of options to adjust different components of your reservation, including adding on an extra day with a Park Hopper option. If you're unable to do so online, call Ticketing at (407) 939-7679, and a Cast Member will be happy to help. You can also visit a ticket window at any of the four theme parks anytime before the end of your third day of use and upgrade to a 4-day
Theme Park Ticket with the Park Hopper option.
I hope this helps, Theresa! See ya' real soon!
Eean