To change the date of birth associated with your Microsoft account, you typically need to follow the account recovery or management processes provided by Microsoft. Unfortunately, if the account is blocked due to age restrictions, you may not be able to access it fully until the date of birth is corrected.
If you prefer to delete the account instead, you cannot delete a Microsoft Marketplace account yourself. You would need to create a support request for account deletion. Ensure that you are the owner or manager of the account and that there are no offers or applications associated with it before making the request.
For further assistance, you can refer to the support documentation provided by Microsoft for managing your account settings and deletion requests.