An invoice that has been sent or paid is never changed. When a job on it is cancelled, FleetKraft makes a credit note instead: its own number (CN-00001, a series for your company), linked to the invoice and the job, for the job's line less any cancellation charge, plus the same tax rate as the invoice.
On a sent invoice the credit note is Applied to invoice: it lowers the Amount due shown on the invoice, the Invoices list, the Dashboard and the customer's copy. On a paid invoice it is Refund due: the money goes back to the customer. Refund it from your bank or payment provider (FleetKraft does not move money), then press Mark refunded and enter the date and an optional reference. It becomes Refunded.
Credit notes show on the invoice page, under Invoices, Credit notes, in the customer's Account (Customers, Account), and read only in the customer portal. Print / PDF opens the customer's copy. Voiding a sent invoice voids the credit notes applied to it.
To give money back for anything else (a wrong charge, a discount), open a sent or paid invoice and press Add credit note: tick lines (all or part of each) or enter a free amount before tax, enter the Reason (printed on the credit note) and press Add credit note. You can never credit more than is still creditable: the invoice total less its other credit notes that are not void.
Void on a credit note cancels one given by mistake (applied or refund due). An invoice with a refund due cannot be voided until you mark the refund done or void that credit note.