When you create a Limit Order, it may or may not get filled right away, or it may get partially filled with the remaining part unfilled.
You may decide after creating a Limit Order that you no longer want it to be accepted by any Takers, and since Limit Orders aren't always filled right away, you can cancel any unfilled Limit Orders at anytime (either fully unfilled or partially unfilled portions).
Cancelling a Limit Order can be done from the Open Orders tab of the Trade screen.
To get started, tap the "Funds" menu item in the bottom navigation bar, then the "Exchange" menu item once on the Funds screen:
After tapping "Exchange" you will be taken to the Markets screen. Tap the trading pair you wish to see your Open Orders for:
After tapping on the trading pair, you will be taken to the Trade screen. From here, tap on the "Open Orders" tab:
After tapping on the "Open Orders" tab, you will see a list of your open orders. If you want to cancel an order, simply tap the Cancel button on the order row (1). You can also cancel all your orders by tapping on "Cancel All" (2):