If your account's birthday information was entered wrong initially and is reflecting you as a minor, you won't be able to change it yourself.
You need to create a new user with correct date of birth as an adult, then sign into this Microsoft account whose date of birth indicating an adult user. Then invite your current Microsoft account to join the family grou. Once joined, go to https://account.live.com/mykids.aspx sign in with the new account (adult one), you should see your "child" account listed. Click on "Edit this child's personal info" And it will let you update the date of birth for this child user.
Once updated, you can then use the "Remove consent for this child's account", then go back to the family safety page, click on this child account and choose "remove from family group".