For orders that are still eligible to be cancelled (i.e. have not been filled) you can use the "Cancel order" button at the bottom of the order details page.
If an order is "Partially Filled" (i.e. some portion of your order has been filled) you will only be able to cancel the unfilled portion. Orders may be "Partially Filled" because only some portion of the shares you were buying/selling were bought/sold on the market.
To access the details page for a pending order, tap the icon in the top left corner to open the sidebar and select "History". At the top of the "History" page any pending orders will appear at the top. Select the desired order to view the details page and the button to cancel will be at the bottom.