How can I change a submenu-item in the WordPress-dashboard to a mainmenu-item?

At first you have to remove the submenu and then add it again. The code should look something like this:

add_action( 'admin_menu', 'register_my_custom_menu_page' );

function register_my_custom_menu_page( ) {
    remove_submenu_page( 'themes.php','nav-menus.php') ; 
    add_menu_page( 'Menus', 'menu', 'manage_options', 'nav-menus.php' );
}

See the WordPress Codex