Cannot reconcile December Cashbook due to changes made to previous month after reconciling. How do I unreconcile to the previous month?

It's good to see you posting here in the Community forum, Lindyl.

I'm here to assist you with unreconciling your previously reconciled transaction from the last month.

If you’re currently using QuickBooks Online Accountant, you can easily undo the reconciliation by following these steps:

1. In the left menu, click Accounting. 
2. Go to the Reconcile tab.
3. Select History by account.
4. Choose the account you want to reconcile and set a date range from the dropdowns.
5. Find the reconciliation on the list.
6. Select View report to open the Reconciliation Report.
7. Review any discrepancies and changes your client wants to make.
8. When you’re ready, select the dropdown ▼ in the Action column, and then select Undo.
9. Select Yes and then Undo to confirm.

For more detailed information on how to undo reconciliation, you can refer to this article: Undo a client’s reconciliation in QuickBooks Online Accountant.

In case you're using a different version of QuickBooks, you can manually unreconcile each transaction to undo the reconciled statement. Here's how you can do that: 

1. Click the Gear icon at the top.
2. Select Chart of Accounts.
3. Locate and choose the account you’re working on, and click on View Register.
4. Select the transactions you want to unreconcile.
5. Keep on clicking the R status until it shows blank or cleared (C).
6. Click on Save.
7. Choose Yes to confirm.

Before starting with any of the above-mentioned steps, I would recommend consulting with your accountant to ensure that all of your records are in order. 

If you encounter any errors or issues while reconciling, you can refer to this article: Fix issues for accounts you've reconciled in the past in QuickBooks Online. 

If you have any other questions or concerns related to reconciliation, please feel free to leave a comment below. We're always here to help you. Have a great day!