You can modify existing orders in the “Orders” widget by right clicking on the order that you want to modify and choosing “Replace Order”. This action will bring up the order modification pop up.
Existing orders that have not been filled or been filled partially are also reflected in the chart widget as a green/red line with the information box. By double clicking on the information box the order modification pop up appears where you can adjust the order price and quantity or set up protective and limit orders. You can also just drag the green/red order line in the “Charts” widget and confirm the price change.