To collect a refund against a payment, simply hit the "Refund" button or quick action on the payment record. You'll then be presented with a dialog:

You can choose to refund up to the original amount of the payment. The refund will then be processed immediately. (Some payment service providers place restrictions on refunds by default. If this is the case, you will need to contact your payment service provider and ask them to enable refunds on your account.)

Multiple refunds can be issued against the same payment, so long as your payment provider allows this and the total amount refunded does not exceed the original transaction value.

Note that your user must have the "Asperato refunds" permission set applied in order to be able to process refunds. This is supplied with the managed package. If your refunds are being declined, you may need to speak to your payment processor - some disable refunds by default, and require them to be manually activated before you can refund payments.