Welcome, Vivienne.
Changing the username isn't quite that simple. However, I have the option of switching to using the name property of the user instead of the username for posts on the forum. Unfortunately, this is a site-wide change. So if I do that, everyone changes their names
Quite a lot of people have used their real names in the name field when signing up (I think it's chrome or firefox autofill

So the forums would look a lot different if I did that.
As things stand right now, I suggest you either live with the username, or create another account. If you do that, post a message to general - we'll be told so we can allow you to post