I am trying to add my own custom header to twenty seventeen theme i have updated the header file but its only working for front page.I want to add same header to single post page also.But i am unable to locate the single post header.
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.
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.
Just rename your edited header file to
header.php so it's called this header on all pages.