All activities stored in Getstream are normalized, thus there is no way for you to update usernames stored within activities. Best practice is to not store data directly in the Getstream activity but store a reference to the data inside your own database (as you are doing right now for the actor field).
{
"actor": "user:$USER_ID"
"object": "post:$POST_ID"
}
Where $USER_ID
is the id of the user in you local database and $POST_ID
is the id of the post (this can be any sort of data e.g. comment, post, like) in your local database). You are also allowed to store extra (custom) fields on the Getstream API.
When you use one of Getstream's integration packages you get this functionality for free. You could have a look at these packages to see how they handle this.