You have to disable 1 click trading. Tools/Options/Trade tab.
How to Place a STOP LOSS and TAKE PROFIT when Trading Forex!
(If you don't disable 1 click trading you will not get the option to close part of the trade and the full trade will be closed when you click on "Close Order")
Then right click on the trade and click "Close Order"
In the window that pops up, click on the figures in the "Volume" box.
Over write with the lot size amount that you want to close. Then click on the yellow close box.
Thanks @Gumrai, your suggestion worked perfectly for taking partial profit on an existing order.
Thanks once again.