The particulars
A summary of section 21(3) of the VAT Act: the words "credit note" (or "debit note"); your name, address and VAT number; the customer's name and address, and their VAT number where they are a vendor; the date; the original invoice it relates to; the amount of the adjustment and the VAT on it; and a short reason. Check the SARS guide for your own case.
Why you never edit the invoice
Your tax invoices are numbered with no gaps, and the customer may already have claimed the VAT on the original. Changing it would leave two different versions of one document. The credit note keeps the record straight.
How it works in Bookkeeping & Invoicing
Open the issued invoice and choose credit note or debit note. The lines come across, you change what needs changing, and the VAT, the customer's balance and the ledger all follow. Issued invoices are locked, so they cannot be edited by mistake.