You can cancel a pending order at any point in time.
Select the from the exchange tab for which you have a pending order and wish to cancel it.
Select the pair and scroll up, Then you have to go on my orders page and you can view your pending orders for that pair under open orders.
Click on the "X" sign next to the order.
Confirm the order cancellation by tapping on "yes" or "no".
You can view the confirmation of the order being cancelled on the bottom of your current screen.