How do I issue credit memo if customer already made a payment on invoice?

We can add the credit memo to invoice payment and adjust the amount received.  Here's how:

1. On the left panel, click Sales.
2. Click the Customers tab, and click the customer's name.
3. Find the Receive Payment associated to the invoice and click it to open.
4. Under Credits, put a check mark on the box of the credit memo you want to apply. 
5. Adjust the Amount received to match the invoice total.
6. Click Save and Close.

You can also check out this article about Credit memos, delayed credits, or customer refunds in QuickBooks Online for additional information.

Feel free to reach out to me if you have any additional questions or concerns, I’m ready to answer them for you. Have a great day!

you obviously didn't understand the question.

the invoice has been paid, they said that. thus, there will be no "receive payment" on the right to click. they want to create a credit memo, so one will not be available to apply. you failed this answer spectacularly. 

Let me guide you on how to create a refund so you can apply it to a paid invoice, Little Colorado LLC.

Refund reduces your bank's balance and offsets the customer's open credit, overpayment, or prepayment. You can use a refund receipt instead of the credit memo. To do this, follow the steps below.

1. Click + New.
2. Select Refund receipt or Give refund.
3. Select the Customer ▼ dropdown, then select the customer you want to refund.
4. Select the Refund From ▼ the dropdown, then select the bank you deposited the payment for the invoice to.
5. Add all products or services the customer returned in the PRODUCT/SERVICE column.
6. Make sure to fill in the service date, quantity, rate, amount, tax, and other fields accordingly, then select Save and close.

Please read this article for more information: Refund for goods or services that didn’t satisfy the customer. Furthermore, you may want to pull up the Transaction List by Customer report and customize it to show all the refund receipts. Visit this article to filter specific data so you'll get the information you need: Customise Reports in QuickBooks Online.

Fill me in if you need anything else about credits and refunds. I'll always make sure to get back to you. Stay safe always!