You can reduce your position size - partially close a position in two ways.
1. Click a Close button and reduce the quantity by pressing a minus. Press SELL or BUY, depending on your position to place a trade.
2. In case the first method is not available or you do not want to use it, you can simply open a new position in an opposite direction on the required quantity.
Opposite positions will be net out at the end of the day unless they have related orders or opened from different accounts. Please read more about Position netting.