Go to Return Request → Requests.
You should see Request Type first before making an option.
If you see a refund request (see the picture above) you can see Refund option for the request (see picture below)
Clicking on Refund will take you to a window where you can manage the refund process (Picture below)
You can choose the refund amount for each individual item.
You can additionally send a note to express the reason along with the refund.
After you are done with these all, Click Process Refund.