How do I convert child Apple ID to adult account?

I created an Apple ID account for my mother from my phone and by mistake set her age to under 16. As a result, the system assigned her an account as a child in Family Sharing, under parental control. My mother is an adult and we need this account not to be treated as a child, because she currently does not have access to basic phone functions.

Please help me change my date of birth or convert my account to an adult account.

[Re-Titled by Moderator]

Original Title: Family sharing