Go to https://appleid.apple.com and sign in
Select Personal Information
Select Birthday or Date of Birth
Edit your birth date
Save the changes
If the birthday cannot be edited, use Apple Support or Family Sharing account settings if the account is a child account
For child accounts, ask the family organizer to update the age through Family Sharing settings or contact Apple Support
If the account is managed by an organization or school, contact the administrator
If the change is blocked, contact Apple Support directly for help