To edit an invoice:

1. Open the invoice from the list
2. Click "Edit"; this is available while the invoice is a draft or finalized but not yet sent
3. Make your changes to line items, customer, dates, or terms
4. Save the updated invoice

Once an invoice has been sent or emailed, or is paid or overdue, it is locked and can no longer be edited. Cancel it and create a new one, or use "Create credit note / correction" for corrections.

Tip: Need to fix a mistake on a sent invoice? A credit note plus a new invoice keeps your numbering and VAT records clean.