I can share with you some information about reconciling accounts in QuickBooks Online, @sbroder.
The beginning date of the new reconciliation depends on the ending date of your last reconciliation. If you've reconciled your account monthly, then all you need to do is to set the ending date to the end of the month.
I've added these screenshots for your visual references.
To learn more about reconciling an account in QuickBooks Online, you can open this article: How to Reconcile an Account in QuickBooks Online.
Just in case you want to undo your past reconciliations, you use this link as your reference.
Lastly, you can open the topics from this link as additional resources while working with QuickBooks or banking in the future, you can as well check out the topics from this link: Find help with bank feeds and reconciling accounts.
I'll be right here to help if you have any other questions. Please feel free to leave a comment below. Have a good day!