Credit an existing invoice
Whenever an invoice is final (so it's no longer in Draft status), you are able to credit it. The option can be found in the menu of Options at the top right of your screen on the Invoice Details page.
When you choose to credit an invoice, that invoice will be duplicated with all invoice lines copied, but the amounts will be in negative.
This new credit invoice will be created in Draft status, allowing you to adjust the invoice lines as necessary.
Whenever you finalise the invoice, it'll receive an invoice number in the same sequence as your regular invoices.
This invoice will be connected to the original invoice, meaning that you'll see a reference to the original invoice on the credit invoice.
If you have enabled invoices to be automatically sent by email to your customer, this new credit invoice will apply to that same rule. If this is disabled, you can always manually send the email under Options.
Create a separate credit invoice
You can also create a credit invoice which is not associated to any other regular invoice, for example when you refund a customer for multiple charges at once.
In that case, you can create a manual invoice in the regular way. Whenever the total amount of the invoice is below zero, the invoice is automatically turned into a credit invoice. The invoice will look exactly the same, but there won't be a "Credit for" field shown.