Thank you for posting here in QuickBooks Community.
I can help you change the ABN number and create an invoice with last year's date. To change the ABN number in your QuickBooks account. I suggest going to Account and Settings and use the 2019 ABN.
1. Click the Gear icon at the upper right corner.
2. Choose Account and Settings.
3. Select the Company tab and go to the Company name section.
4. Click the pencil icon and put the updated ABN number.
5. Hit Done and Save.
For additional details, you can refer to this article: Change your business name, contact info, or ABN in QuickBooks Online. This also provides on how to change the email address.
Once done, you can now create an invoice with last year's date and send to your customer. Please follow these steps:
1. Click the Plus icon at the left panel.
2. Choose Invoice.
3. From the Customer drop-down list, select a customer.
4. Enter their email address.
5. Choose a date in the Invoice date box and Due date.
6. Fill in the other necessary fields.
7. Hit Save and send.