Hi Kathryn,
The most cost effective way for you to add an extra day would be to simply purchase a 4-Day
Ticket for yourself or to upgrade your 3-Day Ticket (if you have purchased it already).
That way, rather than paying for an additional 1-Day Ticket, you will only pay the difference between the price of a 4-Day Ticket and a 3-Day Ticket.
Ticket upgrades must be made in person at the following locations at the Walt Disney World Resort:-
Theme park ticket windows
Guest Relations locations
Lobby Concierge at Disney Resort hotels
If you prefer to update over the phone, please call the Disney Reservation Center at (407) 939-3476 or (407) WDW-DISNEY.
The "
Can I upgrade theme park tickets after they are purchased?" page of the Tickets - Frequently Asked Questions section of the Walt Disney World website describes this in more detail.
Wishing you a magical trip filled with memories which will last a lifetime!
Emma