Let me share some information to ease your confusion, @Kosta.

Changing the Ending date of your reconciled transactions depend on its status. You can still edit it if you haven't completed the process. You can follow the steps provided by @vpcontroller.

However, if it is already completed and you still choose to change the ending date, you can do either of the following:

• You can just leave the ending date as it is. Just make sure to select the correct Starting and Ending date in your next reconciliation. You can do this in your existing QuickBooks Online account.
• The other way is to undo the previous reconciliation that you've made. You can undo it automatically if you're using the QuickBooks Online for Accountant version. You can follow the steps provided by my colleague, @Candice C.
  
• But if you're using the regular version of QuickBooks Online, you'll have to edit the reconciliation status of each transaction one at a time.

Just a heads-up, having an incorrect ending date does not affect any of your balances. Also, this does not prevent you from doing your future reconciliation.  Since your accountant has access to your company, I'd suggest consulting with him or her so you'd be guided on the right steps to follow.

Fill me in if you have other questions. I'm always here to help.