i don't use the website builder, but there are options somewhere to change it
i believe what you are wanting to change is this:
background: #000 url(images/headerbg.jpg) no-repeat left bottom;
that came from your css section in the head.
you can either edit that to a new banner, or rename your new banner to that name, and upload/overwrite the existing.
if your replacement banner is not the same size it might take a bit more work to get things looking good.