You must submit a ticket in order to process the refund.
And during the refund process, certain information is required
1) Steam Transaction ID
2) A screenshot of Steam Transaction page
STEAM Transaction ID and page can be found in Steam launcher.
Please follow the instruction below.
1) Login Steam launcher
2) Fine STEAM account profile menu (nickname) in the top right side of the launcher.
3) Choose "Account details"
4) Choose "View Purchase history"
5) Choose the item that you want to request a refund and click
6) Check the Transaction ID in the order page
7) Take a screenshot and submit the file in the ticket.