For accessing the header layout, You can do so by simply navigating to the Appearance > Theme Options > Header Options.
It contains two types of header layout such as Wide and Boxed. Wide layout occupies entire window width and boxed layout shrink the header and it takes width from Appearance > Theme Options > General Content Wrap settings.
If you want to change the header for individual page. In the below of the page, you can see Page Options. In the Page Options Tab please select the Header.