You received a Credit Note from a Supplier and you want to apply this to the originating invoice. This is important because it helps you to control your spend when matching PO-Invoice-Delivery.
To do so, on Purchasing > Credit Notes, choose the Credit Note you want to use and click on it.
On the bottom of the page, you can see the invoices available to apply this credit. Search for the one that originated this credit and click on the blue button "Apply Credit".
Now, if you visit the invoice page that you just applied this credit for, you will be able to see that its outstanding value has changed.