Notify a User when his/her Role has Been Changed to Specific Role

Write below code in function.php file.

function user_role_update( $user_id, $new_role ) {
if ($new_role == 'editor') { 

 $site_url = get_bloginfo('wpurl'); 

 $user_info = get_userdata( $user_id ); 

 $to = $user_info->user_email; 

 $subject = "Congrats your Role changed to ".$new_role."";
 $message = "Hello " .$user_info->display_name . ", your role has changed on ".$site_url.", congratulations you are now an " . $new_role; 

 wp_mail($to, $subject, $message); 

 }
}

add_action( 'set_user_role', 'user_role_update', 10, 2);