Hi,
Where (which service) do you want to change your legal name?
Edit: If you need assistance with your Microsoft Certification profile, I suggest you ask over here at the dedicated Microsoft Training Support forum, they may be able to assist you further: https://trainingsupport.microsoft.com/en-us/mcp/forum/mcp_prof
or here:
https://trainingsupport.microsoft.com/en-us/mcp/forum/mcp_prof-mcp_incorrectmissing
Best regards,
Leon