To refund a payment, go to your Stripe dashboard.
Click on "Payments" at the top left of the dashboard.
You can then select the payment in question using the box to the left of each line, then click on "Refund" at the top right. You can also click on the three dots to the right of a line and click on "Refund".
Finally, you can enter the relevant information for the refund such as the reason for the refund.