If you want to cancel a booking, simply hover the mouse over the booking and you'lle see a pop up with some info and some buttons as well. Press the cancel button to cancel the booking.
On the next step you will see a drop down menu where you can select the reason why the booking is being cancelled. You can also write a note and decide if you are going to offer a refund to the customers or not.
Once you have selected all the options, press Cancel to complete the cancellation.