How to customize header part of storefront theme in wordpress?

Solution: 

You can easily customize storefront theme from functions.php of storefront child theme.
With the help of storefront_header hook you can do that.

The following code is used to remove storefront header default features-

add_action("after_setup_theme", "storefront_child_header_remove");
function storefront_child_header_remove()
{
    remove_action("storefront_header","storefront_site_branding",20);
    remove_action("storefront_header","storefront_secondary_navigation",30);
    remove_action("storefront_header","storefront_product_search",40);
    remove_action("storefront_header","storefront_primary_navigation_wrapper",42);
    remove_action("storefront_header","storefront_primary_navigation",50);
    remove_action("storefront_header","storefront_header_cart",60);
    remove_action("storefront_header","storefront_primary_navigation_wrapper_close",68);
}

Description: 

When your theme is ready storefront_child_header_remove() function is called and this function call each remove_action function to remove those functionality that you dont want.

You can also easily add your own function in the header part using the same hook "storefront_header" . For example suppose you want to execute your function before storefront_product_search function execution then the following code works -

add_action("storefront_header","my_custom_function",35);
function my_custom_function()
{
   // Your code here
}

Here 35 is the priority that tells the function to execute before storefront_product_search function execution whose priority is 40.