How to make the menu my sticky header in wordpress - php

Question

So far I have the following header on my site:
Right now, when you scroll down the page, the sticky header is the LOGO, Phone numnber and the Request service button in red. How could I change this to use the Services menu below to be my sticky header instead?
I checked the code of the Header.php and found the following lines:
// Sticky header
if(isset($theme_options['enable_sticky_header']) && $theme_options['enable_sticky_header']) {
$header_add_class = ' class="'.esc_attr('sticky-header main-header').'"';
} else {
$header_add_class = ' class="'.esc_attr('main-header').'"';
}
?>
<header<?php echo $header_add_class; ?>>
<div class="container<?php echo esc_attr($header_container_add_class); ?>">
<div class="row">
<div class="col-md-12">
<div class="header-left logo">
<?php
// Center logo
if(isset($theme_options['header_logo_position']) && $theme_options['header_logo_position'] == 'center'): ?>
<?php magnium_header_center_show(); ?>
<?php else: ?>
<?php magnium_header_left_show(); ?>
<?php endif;
// Center logo END
?>
</div>
<div class="header-center">
<?php
// Center logo
if(isset($theme_options['header_logo_position']) && $theme_options['header_logo_position'] == 'center'):
?>
<?php magnium_header_left_show(); ?>
<?php else: ?>
<?php magnium_header_center_show(); ?>
<?php
endif;
// Center logo END
?>
</div>
<div class="header-right">
<?php magnium_header_right_show(); ?>
</div>
</div>
</div>
</div>
<?php magnium_menu_below_header_show(); ?>
</header>
I guess this line magnium_menu_below_header_show() is responsible for displaying the menu, but I really do not know where to search it. Although I looked in functions.php, I could not find it.
I know I could use a plugin, but I just do not want to always rely on plugins if I can make it by myself.