Can I cancel and redo a bank recontiliation?

You need someone with an Accountant license for QBO.  Unless they've just recently changed that.

(Wish they would change it soon.  It's an option in Desktop so I don't see why you QBO users can't do it.)

Yes, you can, Steve.

You can manually undo and redo your reconciliation anytime. In addition to Pete_Mc, you can also invite your accountant and have them undo the process for you. They have the proper tools to undo the reconciliation at once. 

If you wish to undo the reconciliation, you can follow the steps outlined below. Before we proceed, I’d recommend reaching out to your accountant for further guidance. This way, we can ensure your accounts remains accurate after making these changes.

When ready, you can follow along with these steps:

1. Go to the Accounting menu in your QuickBooks Online (QBO) account.
2. Select Chart of Accounts.
3. Find the account you want to undo then select the View register link,
4. Select the transaction you want to unreconcile to expand the view.
5. Review the check column. If the transaction is reconciled, you’ll see an “R.”
6. Select the checkbox until the box is blank. This removes the transaction from the reconciliation.
7. Click Save and close the account register.

You can also see this link for more details: Undo or remove transactions from reconciliations in QuickBooks Online.

Once done, you can now redo the reconciliation process again. In case you run into some issues, I've added this link that can guide you in fixing it: Learn the reconcile workflow in QuickBooks.

You can always count on me if you need more help about reconciling your accounts or any related concerns about QBO. I'll be here ready to back you up anytime. Keep safe and have a good one!

@JasroV   So you still can't actually undo a Reconciliation in QBO, you can just unreconcile individual Transactions?

Any rumors on when they get this obvious feature working?  I get that you've always been able to undo a Transaction, but that may not be the Best Practice if you're trying to correct a Reconciliation error.  The error may or may not be undone when doing it manually, so it will still exist.