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.