All running trades and pending orders will be displayed in the corresponding section on the platform.
Select the trade you'd like to close from the trades list. If you hover your mouse over it, you will see a 'Close' button.
When you click it, you'll see a window pop up with the trade parameters and confirmation button.
If you click the 'Yes' button, your trade will be closed at the market price.
There is another option. Select a trade from the trades list and click on it. After you've done that, you'll see this kind of window:
Here, you can edit your trade parameters or close it by clicking on the corresponding button.