How can I undo the October reconciliation that I have just done?

You can manually undo the reconciliation for the month of October in the Bank Register, Lisa.

Here’s how:

1. Go to Accounting and select the Chart of Accounts.
2. Locate the bank or account, then click Account history.
3. Find the transaction in the list. If it was reconciled, you’ll see an R in the tick ✔ column.
   
   
4. Select the transaction.
5. In the tick ✔ column, select the box with the R until the box is blank.
   
   
6. Select Save.
7. Select Yes to confirm.

You can refer to this article for more details: Undo or remove transactions from reconciliations in QuickBooks Online.

Please note that once you undo the reconciliation and reconcile the account again, any new transactions on that account, including those entered after the original reconciliation, will be included in the ending balance. This may result in discrepancies for the month of October if these transactions aren't properly accounted for.

To ensure accurate account reconciliation, you may consider seeking advice from an accountant.

If there's anything else we can assist you with, don't hesitate to let us know.