You can liquidate your holdings by initiating an outgoing withdrawal by entering in the current value of your account (or something very close to that number based on market fluctuations) as the desired withdrawal amount. A liquidation confirmation and acknowledgment screen will appear. Once you accept the confirmation your positions will be liquidated during the next trade window.
Please note that while your account is liquidating, scheduled transfers will be disabled, as are other deposits / withdrawals. Once your positions have been sold and settled (typically in 3-5 business days) funds will be transferred back to your linked bank account.
If you have additional questions about liquidations, please contact support.