usually all the pages follow page.php, have a look if you have header imported in page.php, if you want a different header menu, create a different header and rename as header-pages.php, assign a different menu here as
<?php wp_nav_menu(array(
'theme_location' => 'page_menu',
'container' => false,
'depth' => 3
))
?>
add this to your functions.php by editing the existing array
register_nav_menus( array(
-------remove this, add the below code only in existing array------
'page_menu' => esc_html__( 'Page Menu', 'website' )
-------remove this, add the above code only in existing array------
));