To issue refunds to customers, you need to do so via your Stripe dashboard, not through Paythen.
From the Stripe Dashboard
Find the charge to be refunded in the Payments overview page (you can get to this by clicking on the "View customer in Stripe" shortcut in your Paythen dashboard too, then click on the relevant payment.
Click the ••• icon to the right of the charge and select Refund charge or click the "refund" icon directly if you can see it.
Enter the amount to be refunded. The default is a full refund. For a partial refund, enter a different amount to be refunded.
Select a reason for the refund. If you select Other, you must provide an explanatory note that is attached to the refund.
Click Refund.
If you have the "Refund email" on via your Stripe settings, a notification will be sent to your customer
👉 Note: Stripe does not refund you the fees on the original transaction. Before you refund the full amount to your customers, consider this and decide whether you'd like to absorb the fee and issue a full refund, or refund your customers the amount with the fees deducted. This depends on your personal preferences and business policies.