Thanks for dropping by here in the Community. I'm here to assist you with any questions you may have about reconciliation in QuickBooks Online.
The ability to undo entire reconciliation is only available in QuickBooks Online Accountant. For regular online subscriptions, you may need to edit individual transactions within the register.
Here are some screenshots for your visual reference:
For additional information on this, please see the following articles:
Please know you can always come to the Community for all of your needs concerning the reconciliation. Wishing you all the best.