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Good day, @cindy48.
If you just started the reconciliation, you can still change the date.
1. Go to the Gear icon and choose Reconcile.
2. On the reconciliation page, click on Edit info beside Save for later.
3. Put the correct Ending balance, Ending date and click save.
If reconciliation was done, the Undo option for a reconciled period is only available in QuickBooks Online Accountant, invite your accountant to your company, and let them undo the entire reconciliation.
In case you don't have an accountant, refer to this article on how to manually unreconciled and undo reconciliation: How to Unreconcile a Transaction or Undo a Reconciliation.
Let me know if you have additional questions, I'm just one post away.