All you need to do is click the "Withdraw" button on your Portfolio page or inside "My Account".
You will be shown how much money is available for you to withdraw. This is the "cash" in your account - either uninvested money or generated from sales of securities you, previously, owned. You can place a withdrawal request and the money will be wired to your domestic bank account. It takes 4-5 business days for the money to hit your account. Any request above $10,000 requires an additional email approval.
There is a $10 fee associated with withdrawals, charged by the US bank, so the cash balance in your account should be at least $10 at the time of placing the withdrawal request.