Nick, you can easily change the date format to dd/mm/yyyy in QuickBooks Online by adjusting the Account and settings. Then, the system updates the invoice and other transactions.
 

Here's how to do it:

1. Click the Gear icon in the upper-right corner and select Account and settings.
2. Navigate to the Advanced tab and scroll down to the Other preferences section.
3. Change the Date format to dd/mm/yyyy.
4. Click Save, then Done.
   

After updating the settings, you can return to your invoices to confirm the new date format. You can refresh the page if necessary.

Please don't hesitate to reach out if you need anything else.