Overrding PrestaShop module instance class

blockuserinfo.php

<?php

if (!defined('_PS_VERSION_'))
	exit;

class BlockUserInfoOverride extends BlockUserInfo
{
	public function hookDisplayNav($params)
	{
		return '<div class="header_user_info"><a>Test</a></div>';
		// return $this->display(__FILE__, 'nav.tpl');
	}
}