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How do you change the end balance of a month that been reconciled, if amount was put in wrong?
You can undo the reconciliation, ronc1.
The Undo option for a reconciled period is only available in QuickBooks Online Accountant. I'd recommend inviting your accountant to your company, and let them undo the reconciliation period at once.
Otherwise, you'll need to undo the reconciled transactions one at a time. You can use this article for reference: How to Unreconcile a Transaction or Undo a Reconciliation.
Check this article for future reference: Fix issues during a reconciliation in QuickBooks Online.
Let me know if you need anything else.