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.
Click on the position from the ‘open positions’ panel. On the close deal ticket, amend the size to the amount you’d like to close. Then click either ‘sell’ or ‘buy’, depending on the direction of the trade, to close the position. For example, if it’s a ‘buy’ trade, you would click ‘sell’ to close.