How to remove dropdown from wp_nav_menu

We can use depth parameter of wp_nav_menu to set how many levels menu walker traverses.

wp_nav_menu( array( 'menu_id'=>'nav', 'theme_location'=>'header-menu' , 'depth' => 1) );

You can create a child theme so that updates won’t override during updates.

Create child theme and copy code from header.php of parent theme then replace the code 

 wp_nav_menu( array( 'menu_id'=>'nav', 'theme_location'=>'header-menu' ) );

with the above code.

More Info:
child theme

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