Hi, Bagya1111.

I'm here to help you apply credit memos to an invoice. 

Ensure that you already created an open invoice to associate it to a credit memo.

Here's how:

1. Choose Customers.
2. Click Credit memos/Refunds.
3. Enter the same customer to an invoice.
4. Fill in the information needed.
5. Tap Save and close.
6. Choose Apply to invoice.
7. Click Done.
   
   

To learn more about recording a credit memo, you may check this article: Record a credit memo or refund in QuickBooks Desktop.

I've also added this article to help you receive and process payments in QuickBooks Desktop.

Fill me in if you have additional questions about applying credit memos to an invoice in  QuickBooks. I'm always here to help. Take care always.