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Voided invoice can't be undone. You'll want to recreate the first invoice and apply the payment to it. Then, you can void the second invoice.
To recreate the invoice:
- Go to the +New icon.
- Choose Invoice.
- Fill in the necessary information.
- Click Save and close.
To apply payments to the invoice, here's what you'll need to do:
- Go to the +New and under Customers choose Receive Payment.
- Select the customer who paid you in the Choose a customer field.
- Enter the date you were paid and the necessary information.
- Click Save and close.
In case you need a handy reference on voiding invoices, check out this article: Void or delete transactions in QuickBooks Online.
If you don't know the exact information, you can check the Audit Log. To do that:
- Go to the Gear icon and select Audit Log.
- Select Filter.
- Use the fields on the Filter panel to choose the appropriate User, Date, or Events filter to narrow the results.
- Click Apply.
Learn more about using the Audit Log Feature through this article: Use the audit log in QuickBooks Online.
You might also want to check out this article for more tips on how to customize your templates including estimates in QBO: Customize invoices, estimates, and sales receipts in QuickBooks Online.
The Community will always have your back if you need anything else in running your business in QuickBooks Online. Have a good one.