how to reverse an accidental refund to a customer from previous tax year

I see that you want to undo a transaction, MattLT. I can help you provide elaborative information making sure that your accounting reports are accurate.
 

If the refund has been manually entered in QuickBooks Online, we can void or delete it. However, as this transaction is from the previous year, it could impact your financial reports and bank balance. I recommend consulting with your accountant before making any modifications to your books. 

Please follow these steps below:

1. Go to the Gear icon.
2. Select Chart of Accounts.
3. Open the bank register holding the refund transaction.
4. Select the refund, then click Edit.
5. Choose Edit or Delete under More.
   
   
   
   
6. Follow the onscreen prompts to confirm the changes.
    

However, if you're using QuickBooks Payments, we can no longer reverse or undo the refund. Thus, you'll have to coordinate with your customer and have an internal agreement to return the funds.
 

Once you receive the money, you can record it as a bank deposit in QBO posted to the appropriate income account.
 

To learn how to close your past fiscal year and gear up for the upcoming one, check out our Year-end Guide for QBO.
 

For further assistance I highly recommend exploring our QuickBooks Live Expert. They are experts in managing various QuickBooks tasks, including handling your reports and transactions.
 

Accurate and efficient bookkeeping plays a vital role in the overall success of any business.  If you have any questions or concerns, please don’t hesitate to click the Reply option. Our team is always available to assist you.