From the "Open Positions" window, right-click within the position row and select "Close position" from the pop-up menu. The Close position confirmation box will appear and the "Amount (Lots)" field will default to the entire position size.  If you wish to close out only part of the position, change this field to the amount of lots you wish to close.  Click "OK".  The position will now be closed at the current market price.

Click on the display below for detailed visual instructions with voice: