All you need to do is select the "Funds" button on your homepage page and then click on “Withdraw Funds”.
You will be shown how much money is available for you to withdraw. This is the "cash" in your account - either un-invested 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. You can only Withdraw funds to your default bank account which was added during fund transfer.
There is a $10 fee associated with withdrawals, charged by the US bank, and thus, the cash balance in your account should be at least $10 at the time of placing the withdrawal request. Any request above $20,000 requires an additional email approval.