Change CTP single's parent from Blog to another page

functions.php

<?php 
// As of WP 3.1.1 addition of classes for css styling to parents of custom post types doesn't exist.
// We want the correct classes added to the correct custom post type parent in the wp-nav-menu for css styling and highlighting, so we're modifying each individually...
// The id of each link is required for each one you want to modify
// Place this in your WordPress functions.php file

function remove_parent_classes($class) {
  // check for current page classes, return false if they exist.
	return ($class == 'current_page_item' || $class == 'current_page_parent' || $class == 'current_page_ancestor'  || $class == 'current-menu-item') ? FALSE : TRUE;
}

function add_class_to_wp_nav_menu($classes) {
     switch (get_post_type()) {
     	case 'project': // CHANGE THIS TO YOUR CPT SLUG
     		// we're viewing a custom post type, so remove the 'current_page_xxx and current-menu-item' from all menu items.
     		$classes = array_filter($classes, "remove_parent_classes");

     		// add the current page class to a specific menu item (replace ###).
     		if (in_array('menu-item-21', $classes))
     		{
     		   $classes[] = 'current_page_parent';
         }
     		break;
     }
	return $classes;
}
add_filter('nav_menu_css_class', 'add_class_to_wp_nav_menu');