You can easily change the due date, Sacha Green. I'll guide you.
Please follow these steps:
- Go to Invoices.
- Select the invoice.
- Click the Send drop-down arrow, then select Edit.
- Hover your mouse over the due date, then change it.
- Click Save.

Concerning the email template, you can customise the invoice. On the Invoice page, you can see the Customise feature on the right pane.
For more information, you can refer to this article: Create invoices in QuickBooks Self-Employed.
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