How to change birthday

Read this support article about how to update the birth date associated with an Apple Account: https://support.apple.com/102473

Edit your birthday on an iPhone or iPad

1. Open Settings, then tap [your name].
2. Tap Personal Information. You might need to sign in to your Apple Account.
3. Tap your birthday to edit it.
4. Update your birthday, then tap the Done button. If you’re changing a birthday from an adult to a child, learn more about converting to a Child Account.

or:

Edit your birthday on the web

1. Sign in to account.apple.com with the account you want to edit.
2. Select Personal Information, then choose Birthday.
3. Update the birthday and click Save. If you’re changing a birthday from an adult to a child, learn more about converting to a Child Account.

If you want to edit the date of your birthday associated with your Apple Account, follow these steps:

...

1. Open Settings, then tap [your name].
2. Tap Personal Information. You might need to sign in to your Apple Account.
3. Tap your birthday to edit it.
4. Update your birthday, then tap the Done ✔️ button. If you’re changing a birthday from an adult to a child, learn more about converting to a Child Account.

...

copied from:Update the birthday associated with an Apple Account - Apple Support

Hi, please see this article.

Edit the birthday on an Apple account

Have a good day.