There may be a scenario in which you need to cancel an entire transaction, this guide will show you how to do this.
With items added to the transaction, at the top-right of the screen there will be the 3 stacked icons, press this and you will have a list of options. Select the one which says [Cancel Order]. This will open up a dialog box like below:
This will ask to confirm whether you wish to cancel the order, if you are happy to do this, press Yes and the order will be cancelled.