Where can I edit Admin Panel Page file

You can add the custom help by adding a hook to the page load e.g. page-new.php would become load-page-new.php

function custom_help_page() {
  add_filter('contextual_help','custom_page_help');
}

function custom_page_help($help) {
  $custom =  "<h5>Custom Help</h5>
              <p>Custom help content</p>";
  return $custom.$help; 
}

add_action('load-page-new.php','custom_help_page'); //New page
add_action('load-page.php','custom_help_page'); //Page, edits, updates etc.

If you don’t want the default help to still display, simply remove echo $help;

Edited to return rather echo the values.