Click the ‘close’ button on the ‘positions’ panel, for the position you’d like to partially close. You’ll have the opportunity to enter a size to close (which will default to the full size of the position). Enter your closing size, the click the green ‘close’ button.
Please note: if 1-click trading is enabled, clicking ‘close’ from the ‘positions’ panel will immediately close the position. In order to partially close when 1-click trading is enabled, you’ll need to open a new ticket, and trade in the opposite direction, making sure to select ‘net off’ on the ticket.