If you need to cancel an open limit order before it gets filled, just follow these steps:
From your homepage, click Advanced Trade to pull up the trading interface.
Navigate to the trading pair that that has your open order.
Scroll down to view your ‘Open Orders’. You will see your open orders in chronological order, starting with the most recent one.
Cancel the order by clicking the red ‘Cancel’ button on the right side. The order will shortly disappear from view.