how to delete a previous reconciliation ?

Let's undo it, prateek2.

We can manually remove the reconciled status of the transactions to remove it from the reconciliation. Please follow these steps:

1. From the Accounting menu, select Chart of Accounts.
2. Find the account holding the transaction. Select the transaction you want to unreconcile to expand it and review the check column. 
3. Click the checkbox with the letter "R" until it's blank. 
4. Tap Save.

Please check this link: Undo or remove transactions from reconciliations in QuickBooks Online. It gives you more detailed steps and some articles on how to handle reconciliation issues.

Let me know if you have further concerns. I'll be right here!