At the bottom of your screen is a horizontal black bar with a button that says "CASH OUT". Click that button, fill out the amount and destination address and click "Withdraw". The withdrawal transaction will be sent to the Bitcoin network in just a few seconds.
In order to withdraw funds, all of your previous deposits must have gotten at least 2 confirmations. This usually takes about 10 or 20 minutes. This is to prevent malicious users from creating fraudulent transactions. A small transaction fee of 0.0001 BTC will be deducted.