I need help on changing my birthday

If running IOS 16:

tap settings

tap [your name], Apple ID, iCloud+ (the line above Airplane Mode)

tap Name, Phone Numbers, Email

tap Birthday

change the year, month and day by tapping the arrow point by the displayed month and year and then tap on the correct date

AppleIDs set-up for a minor (from recollection, for those under 13) cannot be changed.