If you would like to modify the HTML of your header, here's how:
- Go to Design→ Edit Html/CSS in the left navigation
- Under Edit HTML/CSS, select "header.html"
- If you are using the free Shared SSL (e.g. https://yourstore.corecommerce.com) you may also have a secure_header.html which you will need to edit as well.
- Click Publish when done.
Most of the CSS that controls your header should be found in main.css. You can also modify any images that appear in your header by going to Design→ Manage Store Images.