Aloha, Marina,
Welcome to planDisney, and mahalo for surfing by with your question. See the line where the sky meets the sea? It calls me and it sounds like it is calling you, with your upcoming Hawaiian vacation just on the horizon at
Aulani, A Disney Resort & Spa. Marina, usually the accounts for all websites across The Walt Disney Company, including Disneyland, Walt Disney World, Disney Cruise Line, and Aulani, are linked. It is possible that your birthdate is incorrect in your main account, which is why it would be incorrect on your Aulani reservation. I just double-checked in my own My Disney Experience account, and it is not possible to modify my birthdate in my account.
What I would recommend is to start by
calling Aulani and see if they can change it in their system. If they can't, you may need to call Technical and Website Support for assistance. If you have a Disneyland account, I would call the
App & Online Support number and if you have a My Disney Experience account, I would call
Guest Services. This is because it would need to be updated across all of your Disney accounts, and it is the only way to do it.
I hope this helps you, Marina. When visiting Aulani, be sure to enjoy a shave ice with Mickey ears. Everything tastes better when it's Mickey-shaped, especially in paradise!
A Hui Hou (it means until we meet again in Hawaiian),
Keri