How can I undo an invoice I voided by mistake?

Thank you for posting here, @outtatheboxhq-gm.

I'd be glad to share with you the steps in fixing the voided invoice. In QuickBooks Online (QBO), you can only recreate the invoice once voided. You can use the Audit log to see the details when re-entering the voided transaction. Let me guide you on how:

1. Go to the Gear icon and choose Audit Log.
2. Select the Filter button to show only the data you need.
3. From the Date field, set the date the invoice was voided.
4. Choose your name or System Administrator on the User section.
5. Then, click the radio button for Show only these events and check the box for Transactions.
6. From the Show dropdown, select Deleted/Voided Transactions. 
7. Next, look for the voided invoice and click View.
8. Use this information to recreate the invoice. 

For more info about the process, please check out this link: How to undo a voided transaction.

QuickBooks also makes it possible to personalise your sales form to reflect more of your business identity. Please check out this link for guidance: Customise invoices, estimates, and sales receipts in QuickBooks Online.

Keep me posted if you have further concerns about reverting the voided invoice in QBO. I'll be on the lookout for your response. Have a good one.