Refunds always cover the full invoice. Partial refunds are not available. To refund a paid payment, click "Refund" on its line in the order or in the customer page, then confirm. To refund several payments at once, select them on the Payments page and use the group action "Refund invoice(s)". By default, your customer receives a confirmation email once the refund goes through.
Refunds are processed through your payment provider (Stripe or GoCardless). A card refund generally appears on the customer's statement within 5 to 10 business days, depending on their bank. For a SEPA direct debit refund, the timing also depends on the customer's bank.
If the refunded payment was part of a subscription, the subscription remains active unless you cancel it separately. A refund does not stop the next charges: cancel the subscription too if your customer should not be billed again.