You can make amendments to an upcoming individual order, or cancel it, all the way up until the order is scheduled to be sent.
Change an order
Head to "Orders" then "Recurring Order"
Identify the order you would just to change and click the arrow
You change the quantities you want to receive, add or remove products from the order
Click "Save Changes"
Cancel an order
Head to "Orders" then "Recurring Order"
Identify the order you would just to change and click the arrow
Click "Cancel Order"
If you needed to cancel an order after the recurring order has been sent out to the supplier, you'll have to contact the supplier directly. It's then up to their discretion/policy on how they deal with requests to cancel orders that have been placed with them already.