The refund can be processed by going to the sales section where on the customer's sales page you'll get an option for the refund on the top behind the three vertical dots in the Pabbly account.
In Pabbly Subscription Billing releasing the amount for a refund depends on the payment gateway through which the transaction has taken place. Like for Stripe, you can directly refund the customer from the Pabbly account itself. Also, it provides the option to record a refund for offline payment & payments with other gateways. Further to know more, please follow this doc - https://www.pabbly.com/subscriptions/docs/features/#section9