How do i remove a past month reconcile and re do it? as per all the research only accounting firm can do the undo option - but then how to add in accounting firm?

Hello and welcome to the Community forum, sanghani-mehul-g.

Let me share some information about undoing your previous reconciled transactions in QuickBooks Online.

Yes, you're right that the option to undo a previous reconciliation is only available to QuickBooks Online Accountant (QBOA). We'll need an accountant to undo the reconciliation at once.

Here's how to invite your accountant:

1. Go to the Gear icon.
2. Select Manage User under Your Company.
3. Click the Add user button at the upper-right corner.
4. Select Company admin. Make sure to tick the This user is my accountant box.
5. Hit Next at the bottom part of the page.
6. Enter their correct information. ( first name, last name, and email address).
7. Then, Save.

We can take a look at these sample screenshots for additional reference: 

Once done, an email will be sent to your accountant. Let them to know that they'll need to accept your email so they can help you undo your past reconciled transaction and other accounting side that needed to be fixed.

Note: we can manually unreconciled each transaction but, we're unable to undo the entire reconciliation at once. That's the reason why we'll need an account to make thing possible.

Let me include the steps on how to do it manually. Here's how:

1. Go to Accounting on the left panel.
2. Find the appropriate bank.
3. Under the Action column, click Account history.
4. Under the Check column, click the box repeatedly to change the status from C/R Tto blank which means the transaction is changed to unreconciled one.
5. Then, Save.

You can check this article where you can gather details and learn how reconciliation works in QuickBooks Online: Learn the reconcile workflow in QuickBooks. 

Post again if you have other questions about QuickBooks Online. I'll be here to help you right away. Take care!