Hi. I'm Brian and I'll try to help.
Unfortunately, Microsoft makes it virtually impossible in some countries to change your own birth date if the current date makes you look like a minor. That said, Microsoft provides this guidance: https://support.microsoft.com/account-billing/837badbc-999e-54d2-2617-d19206b9540a. If this doesn't work, try using the Help feature in the account when you log into https://outlook.live.com/.