Welcome and thanks for visiting the Community, @lalit-dhanker.
If the credit note was already applied to payment, all you need to do is to delete the payment. This will unlink the credit, which can now be used for future invoices.
The steps to do this are pretty quick and easy. Here's how:
In QuickBooks Online (QBO), go to the Sales menu at the left pane to get to the Customers page.
Click the customer's name and proceed to the Transaction List tab.
Locate and select the payment to open it.
Tick the More option at the bottom and choose Delete.