You can close an open trade by first finding it in the Terminal window under the Trade tab.
Once you have found the trade you want to close, you can either double click on it and click the big yellow Close button, or right click on the order and click on Close Order.
You can modify a trade by first finding it under the Trade tab in the Terminal window.
Once you have found it, you can double click on it and select Modify Trade from the Type drop down menu and carrying out your modifications.
Another method is to right click on the trade and select Modify or Delete Order.
To partially close an order, double click on the order, enter the volume you wish to close at the top, and then click on the yellow Close button at the bottom.