Change()

// Example code
Order stopOrder;
stopOrder.StopPriceChanged = stopOrder.StopPrice - 4 * stopOrder.Instrument.MasterInstrument.TickSize;
private void OnExecutionUpdate(object sender, ExecutionEventArgs e)
{
    // Change the stop order if an execution results in a long position
    if(e.MarketPosition == MarketPosition.Long)
        myAccount.Change(new[] { stopOrder });
}