STEP 1
Go to Invoices -> click the View button. Check if the invoice has registered payments, in this case you can’t storno the invoice. To storno the invoice, delete the payment first (How to delete a payment).
STEP 2
Go to Actions -> select Create Credit Note. The system will automatically create a new invoice with identical but negative value.
Validate the invoice and It will automatically take a number, like any other invoice.
After the validation, it will automatically add payment to both your negative invoice and the original invoice. So, in the invoices/payments reports you will have two invoices/payments that sum up to 0.
ATTENTION: At the bottom of the invoice is a link to the storno invoice that will help in their easy identification.