The site's default header is the header automatically assigned to new pages and sections.  Also, it is the header assigned to all the old pages that have not had their headers changed to something other than default. 

1. Login.
2. Click "Change Site's Look and Feel" --> "Change Site's Colors and Images."
   
   
   
   
3. A window should open.
   
   Select a color: Choses the pages color scheme.
   Select an image: Chooses the header image.
   
   
   
   The site's default header has been changed.

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Problem #1: "I changed my default header, but some of my pages still have a different header?"
Answer: You succeeded in changing the default header, however you probably have some pages that have had their headers changed to something other than the default.  Click here to learn how to set those pages back to the default header.