You can cancel any unmatched order you placed on the Portfolio page, which you can access by clicking the Portfolio tab. If your order was already matched you cannot cancel it anymore. If you wish, you can close out your position by clicking on the Sell Order button and following that flow.
Note: Once your order is matched with someone else’s and it becomes a trade, you cannot modify it anymore.
If you want to close out your position, you can click on the Sell Order button, which will guide you through the process. You might lock in a profit or a loss by closing out your position depending on the ongoing price of the shares of your position.