How to add custom header to Twenty seventeen single post in wordpress

Firstly, if you have not already created a child theme of the Twenty seventeen theme, i’d recommend doing that first as any updates to the theme will overwrite your changes. See here regarding child themes.

Copy header.php into your child theme and then add your custom header code into that file.

There is not separate header files for each post type. It’s all contained in the one header.php file.