How to change the header and edit other Single Post settings?

On Appearance > Customize > Blog panel there is a Settings Page Slug field. This field should contain the slug of the page which serves as a template for all blog posts.  All blog post will inherit settings and shortcodes of the page referenced via this field.

                                                 

In order to change elements on blog posts, please locate the page with the slug entered in Settings Page Slug field and edit it.