You can click the link below to follow the step by step guide:

https://beyondbeaver.com/wordpress-different-main-menu-on-different-pages/

However, all you need to do is edit this code below and place it within the functions.php file of the theme via FTP: 



// Conditionally change menus
add_filter( 'wp_nav_menu_args', 'bb_wp_nav_menu_args' );
function bb_wp_nav_menu_args( $args = '' ) {
	// change the menu in the Header menu position
	if( $args['theme_location'] == 'header' && is_page('1159') ) { 
		$args['menu'] = '32'; // 32 is the ID of the menu we want to use here
	}
	return $args;
}



The only areas you will have to change are in bold below:



Your chosen navigation shortcode: EXAMPLE ['theme_location'] == 'header'
The ID of the page you're editing: EXAMPLE is_page('1159')
The menu ID: EXAMPLE ['menu'] = '32'