Your username is visible only in your profile page URL, as well as on all your post URLs, provided that those posts are not published in any publication.
Changing your username
- Click on your avatar in the top-right corner of the page.
- Click Settings.
- Click Edit username in the Account section.
- Enter a new username and confirm.
When you change your username your existing posts will be re-directed when people visit old URLs, but your profile will not automatically re-direct. You should update any links to it from other places.