I am not able to undo previous bank reconciliation although I logged in using accountant account, how I solve that? I am using QBO Essential

I'm here to ensure you're able to undo your previous bank reconciliation, @tfhm2000-yahoo-c. 

There are three levels of access in QuickBooks Online Accountant (QBOA). These are Full, Basic, and Custom. The Full Access user has access to accounting features and books, such as the Undo reconciliation tool. Your user permissions may have been set to Basic or Custom. That's why you're unable to see this option on your end. For more details, check out this article: Different Levels Of Access For Firm Users. This link is specific to the UK region, but it also applies to your country. 

First, I'd suggest contacting the master admin of your accounting firm. You can request them to change your user permission from there. Once granted, let them perform the steps below. 

1. Go to Team from the left menu. 
2. In the Access tab, locate and select the team member.
3. Go to the Firm Administration and Books tab.
4. Choose Full from the Access drop-down. 
5. Select the Client Access tab.
6. Check the box beside the client's company. 
7. Click Save.

The screenshot below shows you the last five steps. For the detailed instructions, visit this article: Changing Team Member's Access. 

Once done, you'll be able to see the Undo option from the History by account page. Here's a sample screenshot of how it looks like in QBOA. 

If your master admin doesn't grant you Full Access permission, I recommend logging into your Essentials account. Then, let's go to the Account History page to manually unreconcile each transaction from your previous reconciliation. You'll first have to print a reconciliation report to serve as your reference. Once done, proceed with the steps below.

1. Go to Accounting from the left menu and select Chart of Accounts.
2. Locate the appropriate account.
3. Choose Account history from the Action column. 
4. Select the transaction.
5. Go to the Check (/) column and click the box many times until it is blank. 
6. Click Save. 

For the detailed instructions, see Step 2 through this article: Undo And Remove Transactions From Reconciliations. 

After that, reconcile the account again to correct the previous reconciliation period. It's already balanced if the difference is zero. 

You can always visit this resource hub: Reconcile Workflow. It contains in-depth details about starting, fixing any differences, and completing the reconciliation process. Just click the links beside QuickBooks Online. 

Please let me know if you need anything else. I'll gladly help, @tfhm2000-yahoo-c.