You can choose to partially close a position. This means that if you open a position with a volume of 10 USDJPY, you can choose to close a portion of that volume. For example, you can choose to close 3 lots and leave a volume of 7 lots open.
You can do this by double clicking on an open position, allowing the order window to pop up. Under the 'Volume' field you can override that with the desired volume you wish to close, and click on the Close button highlighted in yellow.