From the game page, go Behind the scenes. Each attending skater who paid for this game will have a link under the Payment type column with the text View payment item. Clicking that link for the skater you're refunding will take you directly to that charge item in your Stripe dashboard. Click Refund Payment to process the refund.
If you also want to remove this skater from this game, that's a separate process. From that game's Behind the scenes page, hit the red Remove button for that skater.