I have copy nectar folder in my child-theme and try to modify the page-header.php file in helpers which is inside nectar folder but the changes are not reflecting  and if i directly modify changes in the page-header-php file in salient theme itself they reflect . 

Hello, 
I actually repeat the same question, the link is focusing on the Salient elements, not the page-header (for post- single template). I'm trying to override the post single template fullwidth header by adding the excerpt under the post title. I noticed this is coming from the file page-header in the nectar/helpers/page-header because when I edit it from the Salient theme folder, the changes appear. I tried different ways but never found a way to override it in my child theme. 
Unfortunately, the developer documentation is very poor :/
Hope you can shed light on that!
Thank you for your help, 
Charlotte

Hey Tahir, 
Thank you! It's working!! 
Best,

Do you just copy this function into your functions.php file of your child theme and edit it there?

The referenced docs are not covering this specific case and the support provided is vague. We just need to "modify page-header.php file in child theme". This is typically done by using the exact file path a specific file alternate file path with other themes I have used in the past. 

The part Tahir mentions with copying the function from the page-header.php template into the child theme functions.php is quite vague. I managed to hack a solution in the end in my functions.php file but this is not ideal compared to a normal template override method. A better set of documentation is at the least requested.