Hi, please can you provide me with information how can I apply a credit note to invoice which already is showing as paid? Many thanks in advance, Karolina

Hello there, Karolina.

Let me show you how to add a credit note to a paid invoice. 

Let's open the payment transaction so we can link the credit note. You can follow these steps:

1. Go to the Sales tab and select Customers.
2. Select the customer name. Find the payment transaction and click it.
3. On the Receive Payment transaction, select the credit you want to apply.
4. If the amount of the credit note is equal to the invoice payment amount, change the Amount received to zero. Otherwise, you'll have to change it to the actual amount you received without the credit note.
5. Click Save and close.

You can also refer to this screenshot and article for more details: Enter and apply credit notes.

Please post again if you need more assistance with this. Have a great day!