You can delete invoices from the Invoices page, accessible from the Sales menu.
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To delete an invoice
To delete an invoice
In the invoices list, find the invoice you want to delete and click it. The Invoice page appears, showing the invoice details.
Click Delete (at the bottom of the page).
A confirmation message appears.
Click Yes to delete the invoice. The Invoices page reappears.
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Can't delete an invoice?
If the Delete button is inactive, it means the invoice has a payment against it so can't be deleted. You'll first need to delete the payment (click the payment to view its details then click Delete).