SSN

Hello.
I have used att mibile phone for about an year.
When I started to use the phone, I didn't have SSN.
Because I was an international student and I am.
Now I got SSM, I want to enter SSM to my account but I can't find where on line web site or app.
Can anybody help me to enter SSN to my account?
Thank you in advance.
Taeseon Yoon