public function updateFormField($fieldName, $fieldType, $fieldValue) 
    {
        if ($fieldName == '')
            throw new Exception('Field name not specified');
        
        if ($fieldType == '')
            throw new Exception('Field type not specified');
        
        if ($fieldValue == '')
            throw new Exception('Field value not specified');
        
        //build URI
        $strURI = Product::$baseProductUri . '/pdf/' . $this->getFileName() . '/fields/' . $fieldName;

        //sign URI
        $signedURI = Utils::sign($strURI);
        
        $postData = json_encode(array(
                            "Name" => $fieldName,
                            "Type" => $fieldType,
                            "Values" => array($fieldValue)
                          ));

        //get response stream
        $responseStream = Utils::ProcessCommand($signedURI, 'PUT', 'JSON', $postData);

        $json = json_decode($responseStream);
        
        if ($json->Code == 200)
            return $json->Field;
        else
            return false;
    }