Theme Horse

Hi themehorse3,
First of all create a child theme folder name attitude-child in the theme folder directory and then create two files style.css and functions.php and under style.css file add this code

/*
 Theme Name:   Attitude Child
 Theme URI:    https://www.themehorse.com
 Description:  Attitude Child customization
 Author:       Theme Horse
 Author URI:   
 Template:     attitude
 Version:      1.0.0
 Tags:         
 Text Domain:  attitude
*/

@import url("../attitude/style.css");

/* =Theme customization starts here
-------------------------------------------------------------- */
[rest of custom style css]

and under functions.php file first unhook the functions and then only customise the code.
For example to unhook the functions and over ride the parent theme for page 404.php

This is just an example, You need to customise the code in the same way for header-extension.php, if you are unable then you need to hire a developer to fix it.

unhook functions

// Unhook default Thematic functions
function unhook_thematic_functions() {
    // Don't forget the position number if the original function has one

remove_action( 'attitude_404_content', 'attitude_display_404_page_content', 10 );

}
add_action('init','unhook_thematic_functions'); // removes the header content by using hook attitude_header

add_action( 'attitude_404_content', 'attitude_child_display_404_page_content', 10 );
/**
 * function to show the footer info, copyright information
 */
 
 
function attitude_child_display_404_page_content() {      ?>   
<div id="content">
		<header class="entry-header">
			<h1 class="entry-title"><?php _e( 'Error 404-Page NOT Found', 'attitude' ); ?></h1>
		</header>
		<div class="entry-content clearfix" >
			<p>Thank you</p> 
		</div><!-- .entry-content -->
	</div><!-- #content -->
  <?php 
}

Thank you!