IG Trading platform: To partially close a deal go to the ‘positions’ window, and click on the market name. You will then need to select the opposite direction to your current trade, and fill in a size smaller than the position you currently have open. Be sure to select ‘net off’ on the deal ticket. This will then partially close your open position.
Old platform: To partially close a position click on the asset name in the ‘open positions’ window. Fill in a size smaller than you currently hold and click the buy or sell button as appropriate.