Reconciliation is an essential part of any small business. I know how important it is to ensure your reconciliation is accurate, and to have the ability to make any changes to your transactions once the reconciliation is complete. I'd be happy to explain this process in QuickBooks Online.
You may not need to undo your reconciliation, depending on your circumstances. Take a look at this article to see if you can avoid undoing a reconciliation. As explained in the article, if you're connected to an accountant in QuickBooks Online, your accountant can undo or delete completed reconciliations.