Can I undo a change I made yesterday that is impacting on my current return

We can perform some troubleshooting steps to undo the transactions in QuickBooks Online (QBO), Hazel7.

If you're trying to undo the transactions from your reconciliation, we can edit and unreconcile them individually. Here's how:

1. Go to the Gear icon on the top menu.
2. Choose Chart of accounts.
3. Find the account that has the reconciled transaction and click View register.
4. Find the transaction. If the transaction is reconciled, you’ll see an “R” in the checkmark ✔ column.
5. Choose the transaction to expand the view.
6. In the checkmark ✔ column, select the box with the “R” until the box is blank. This removes the transaction from the reconciliation.
7. Press Save and close the account register.

You can check out this article for more details about unreconciling transaction: Undo or remove transactions from reconciliations in QuickBooks Online. Then, you can reconcile your accounts to match your bank and credit card statements again. 

If you need to start over, I'd suggest reaching out to your accountant to undo the entire period at once using QuickBooks Online Accountant. In case you need to undo a voided transaction, refer to this article for the steps and details: How to undo a voided transaction.

Moreover, you can refer to this article to help you close out your current year and prepare for the new one in QuickBooks: Year-end Guide for QuickBooks Online.

The Community will always have your back if you need further assistance with reconciliation or any QuickBooks-related, please let me know by leaving a comment below. I'd be more than willing to lend you a hand.