You must enter the order you want to cancel and click on the "Cancel order" button found at the end of the order summary

Currently it is not possible to cancel an order placed with the third-party supplier. You must contact him to cancel it. 

Don't worry, we are working to have this functionality very soon. 

You find it in "Menu" > "My suppliers" > click on the supplier you want to see the list of, > click on "Orders" and you will find the list of all orders made previously. From there you can also repeat an order.