Hey Claudia!
Welcome to planDisney and thank you for bringing your question to us!
The mistake you’re describing happens quite often. No need to worry! You will be able to make this upgrade and enjoy your visit. But, the upgrade from a child to an adult
Theme Park Ticket, will need to be made in person at the
Disneyland Resort Main Entrance Ticket Booth. A Cast Member will be happy to assist you. Please be prepared to pay the price difference in full.
Also, you’ll need to make a
Theme Park Reservation ahead of your visit. That's because you’ll need that reservation in addition to your valid theme park admission to gain entry. You can make reservations up to 180 days in advance. And your reservation should carry over once the ticket upgrade is made. Here’s how to make or modify a Theme Park Reservation, log in to your Disneyland Account and select "My Disneyland." Then, click "Ticket and Reservation Details" and select "Make or Modify Park Reservations." You’ll be able to book a Theme Park Reservation for your desired date.
Enjoy your trip! If you have any more questions, bring them to us here at planDisney!
Tiffanie