Custom Admin menu name

Hello i need to change the admin label name becouse “MailPoet” create some confusion and difficult for my client. I need to rename as Newsletter.

I try to put this on functions.php but nothing change…how can i do that? Thanks

function customize_post_admin_menu_labels() {
 global $menu;
 global $submenu;
 $menu[21][0] = 'Newsletter';
 echo '';
 }
 add_action( 'admin_menu', 'customize_post_admin_menu_labels' );

Where [21] is position on menu of MailPoet Label

Thanks