To cancel a paid subscription and issue a refund, navigate to your Subscribers tab.
Locate the paid subscriber by entering their email address in the Search bar:
Click on the three dots next to their name and select "Edit Subscription":
You'll then have the option to cancel their subscription with a pro-rated or full refund.
Once the paid subscriber is canceled, the reader will be notified about via email about their refund and be downgraded to a free subscriber. They'll receive free posts but won’t have access to paid-only posts or comments on paid-only posts.
To learn more about Substack's refund policy, read this article.