I have list of categories, and they showing in top menu
<?php $_menu = $this->getHtml('level-top') ?>
Magento displays all of categories, i need to display only a child categories with root category id, for example 23, i find in Mage_Page_Block_Html_Topmenu
$this->_menu = new Varien_Data_Tree_Node(array(), 'root', new Varien_Data_Tree());
but i dont know how i can change root category. Help me.