Firstly navigate to the Order History page of the Partner Hub and locate the order you wish to cancel.
Next, expand that order to display the actions you can take on the order. One of these actions is the red cancel button.
If the retailer allows cancellations you will see the following message:
The cancellation is then clearly shown in order history, including when the order was cancelled and by whom.
How do I know which Retailers allow cancellations?
You can find this in the Brand details section of the Hub, for example:
It is also important to note that some retailers only allow cancellations for a small window. For example Amazon allow cancellations only in the first 15 minutes post sale.
What if the Brand/Retailer does not support cancellations?
You can always get in contact with the retailer to see if they will process the cancellation. If they agree make sure they mark the order as cancelled in the Hub so that your reporting is correct - direct them to this article if they are unsure how please see this article.