How to change the header and edit other Single Post settings?
All single posts inherit the settings from their template page. In order to edit excerpt text and header background image, please locate this page on All Pages list, and proceed to follow instructions from our article.
In order for this page to work as a template page for single posts, make sure to add its slug to Settings Page Slug field in Appearance >Customize >Blog panel.
For more info and detailed instructions please refer to our Online Theme Documentation.