Greetings Taekyu!
Welcome to planDisney and thank you for bringing your question to us!
The mistake you’re describing happens quite often. Please do not fret! You’ll be able to upgrade your
Theme Park Ticket from a child to an adult and enjoy your visit. But, this upgrade will have to be made in person at the
Disneyland Resort Main Entrance Ticket Booth. Upon arrival, please head straight to the Ticket Booth instead of the Main Entrance turnstiles and be prepared to pay the price difference in full at that time.
Also, you’ll want to make a
Theme Park Reservation ahead of your visit. You’ll need that reservation in addition to your valid theme park admission to gain entry. You may make reservations up to 120 days in advance. And your reservation should carry over once the ticket upgrade is complete. 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 visit to “The Happiest Place on Earth!”
Tiffanie