You can withdraw the amount within your free margin limits. In other words, you can only withdraw funds not involved in trading, indicated as free margin.
What is free margin?
The "available" amount of funds shown in your Personal Area is a free margin (constantly changing) that can be withdrawn anytime:
However, it's not advised to withdraw while holding open positions, as there is a possibility that if you experience loss, it might result in a Stop Out. In such a case, extra funds will be helpful to prevent automatic order closure.