You may delete order by clicking on line with particular instrument in window with already placed orders (open this window in “Portfolio” --> "Orders"), the chart window will pop up while the additional window with fields for order parameters and two buttons will appear in lower half of the screen:
In this window you may edit order parameters by clicking on "Edit pending order" button. You may also delete the order by clicking on "Delete pending order" button.
OCO orders and activated orders may be modified or deleted in the same way.