I need help on how can I undo all the previous reconciliation, I need to reconcile the year 2019. They only reconcile 2020.

Thank you for reaching out to the Community, @martellabertiz-d. I can help you fix a reconciled account and keep your books in order.

Before we begin, please note that undoing the entire reconciliation is only available in the QuickBooks Online (QBO) Accountant subscription. If you have a different version, I suggest you contact your accountant, as they can undo the entire period at once.

If you currently use QBO Accountant, here's how to undo reconciliation:

1. In the left menu, click Accounting. 
2. Go to Reconcile tab.
3. Click 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, click the dropdown in the Action column, and then select Undo.
9. Hit Yes and then Undo to confirm.

For more information on how to undo reconciliation, please see this article:  Undo a client’s reconciliation in QuickBooks Online Accountant.

If you use a different QBO version, you can undo a reconciled statement by manually unreconciling each transaction. 

Here's how:

1. Go to Gear icon, then select Chart of Accounts.
2. Choose the account that has the reconciled transaction. Then click View Register.
3. Select the transactions you want to unreconcile.
4. Keep on clicking the R status until it shows blank or cleared (C).
5. Click on Save, then Yes to confirm

Finally, you can learn the reconcile workflow in QuickBooks and view, print, or export a reconciliation report.

If you have any additional issues or need clarification regarding reconciliation, please let me know. I'm available 24/7 to help you out.