It's in either of these:
- If the banner is text, that comes from Admin > General > "Q&A site name".
- If the banner is an image like it is here, that is under Admin > Layout > "Show a logo image in the page header". Tick that then put a URL to an image on your site, and add the width/height.
If you need something even more customized, the banner is output from the logo() function in the theme (qa-theme-base.php or qa-theme.php in your theme folder).