Heartbeat Payments is built with Stripe, so in order to process a refund for a member, you will need to handle this in your Stripe Connect account directly.
How can I access my Stripe account?
To access Stripe directly from Heartbeat, go to Settings > Payments > Configure Payments and click on "Visit Stripe" on the top left.
How do I refund a payment?
To process your refund, search for the customer email in Stripe and then follow Stripe's refund guide. Your member will see the refund as a credit approximately 5-10 business days later, depending upon the bank.