In the order details page, there are a few buttons/menus to trigger cancellation or refund process.

Supported Refund Methods

Steps for Initiating Full / Partial Refund

Go to your order listing page and click on an order you wished to refund.
From the order details page, click on "Refund" button
Insert full number of items (you may optionally unselect some if you are doing partial-refund) and insert reason of refund. Then click "Refund" button
A confirmation message will be shown "Refund is processing" and you will be redirected back to the order details page with additional tracking info in the order timeline.
After some times, the order timeline will be updated with success refund status as follows

Go to your order listing page and click on an order you wished to refund.
On the details page, click on More Actions > Cancel Order
Ensure "Now" option is selected in the Refund Payment. Optionally, adds in "staff note". And then click "Cancel order" button
An acknowledgement will be shown at bottom "Order cancelled" with notation of "Pending Refund" at order timeline.
After some time, the order will be updated with refund success in order timeline