In Tryton a credit note is an invoice with negative quantities. You can create it manually or let the system create it based on an existing invoice.
In order to credit an existing invoice you should use the Credit action in the action menu:
The action will be executed with the current selected invoices, allowing you to credit multiple invoices. On the title you will see the invoice numbers that will be credited.
If the with refund option is checked the credit note will be used to pay the original invoice. Otherwise the credit invoice will be in draft state which allows it to be edited before posting.
Every credit invoice line will have its origin field set to the credited invoice.