How to update a custom field value using code ?

-- HikaShop version -- : 2.3.0
-- Joomla version -- : Joomla! 3.3.1
-- PHP version -- : 5.4.29

Hi,
i created an order table custom field called "firstpayment". (text type) attached screen capture.
i want to update its value (according to some calculations im doing in the payment plugin) and save to the order table when order is saved.

here is the relevant part of the code from my payment plugin im using to achieve this.

class plgHikashoppaymentMyPayment extends hikashopPaymentPlugin
{

              function onAfterOrderConfirm(&$order,&$methods,$method_id){
                   
                     $order->firstpayment  = 100;
                      .....
                    $this->modifyOrder($order,$order_status,$history,$email);
              }

}

Hi,
after reviewing this post
www.hikashop.com/forum/payment-methods/8...m-fields.html#160883
i moved the code assigning new values to $order->firstpayment custom fields ,
to be triggered under onBeforeOrderCreate function.
it solved it.

Thanks