Canceling an order can be accomplished on the order detail page. The supplier will receive an email about the cancellation.
Canceling an order will take the follow steps to complete.
Go to the orders page and select the order you'd like to cancel:
On the orders detail page, the cancel order button is in the upper right corner:
When you hit cancel, a prompt opens to send an email about the cancellation to the supplier. The ability to add notes about the cancellation are optional, and will also be sent to the supplier.
Regardless if you canceled an order, the ability to issue a follow-on will always be available.