I think you'll find it's a bit more complicated than that...you can't just update the attribute, bu thave to update the email itself.
So, try the following sequence to see if you can get it to work:
Use
Email Interaction Get Message to get the cookie using Interaction1
Use
Open Message By Cookie to open the email into its component parts
Use
Parse Email Body using GetHead(lebBody) for the email body you want to retrieve (if there is more than one body attached to the email, you may need to check how many with a loop involving GetCount(lebBody) and looping through to check each of the bodies in thelist)
Use you current functionality to check for SSN in the sData variable that comes out of Parse Email Body and create a modified string
Use
Make Email Body to create a new email body containing the changes
Create the new lebBody, containing however many bodies you need to put back (make sure if you have multiple to begin with that you build it all back up in the same order they were to start)
Use
Email Interaction Update Message to update the original email associated with Interaction1, using all the original variables from
Open Message By Cookie, which will replace the old lebBody with the new lebBody you created.
I haven't tried all of that...but I think it's the route you'll need to go.