Hi Rebecca - welcome back to planDisney!
You're suffering from a pretty standard ailment that I struggle with regularly - wanting more time at the Walt Disney World Resort! Since it looks like you might be visiting from across the pond, I think it's a fantastic idea to make your trip as long as possible given the travel time it takes to get you here. The great news is that you've left yourself plenty of time to make those changes now if you'd like to!
Rebecca, my suggestion is to work backward and try to update your reservation via the original booking method. So, if you booked directly through the Walt Disney World Resort website, but aren't seeing an option in My Plans online or in the app to update your reservation, you should
call and speak to a Cast Member to learn about your options. Alternatively, if you used a travel professional or other third party to make your reservations, you should start there.
Another option you might want to consider is making a new reservation for a
different Disney Resort and enjoying a split stay! The Disney Resorts Collection has so many unique options and many Guests love to spend part of their trip experiencing a different Resort. If this sounds exciting to you, you're welcome to simply make a separate new reservation for those additional nights. I always like to call in and have a Cast Member notate my account that the two reservations are linked, but that's not required.
No matter what you decide to do, you're in for a magical trip. There are so many new and exciting things to experience, including one of my new favorite attractions,
TRON Lightcycle / Run Presented by Enterprise located at Magic Kingdom Park!
Have a magical day!
Ryan J.