Welcome and thanks for visiting our forums. You've got the right place to help you with your reconciliation.
If you're referring to the Statement Ending Balance of the reconciliation page, we're currently unable to change this once the reconciliation is completed. Take it easy, I've got a workaround for you to complete your work. In QuickBooks, you have the option to undo your reconciliation manually from the register. Let me show you how to do that:
Click the Gear icon.
Tap Chart of Accounts.
Find the account you want to review.
From the Action column, click Account history.
From the reconcile status column (indicated by a check mark), click the box multiple times to update the codes on the transaction:
C - Cleared
R - Reconciled
Blank - Not cleared or reconciled
Here's how it looks like in QBO:
Once done, you can now reconcile the account again with the correct statement year. You may want to check these related articles for more reference: