To apply a credit note to an invoice in Xero, please follow these steps:
Log in to your Xero account.
Go to the Invoices page.
Click into the invoice you wish to apply the credit note to.
Click on the three dots on the right hand side at the top.
Select Create and Apply Credit from the dropdown
This opens up a new window:
Selected the credit note already in the system or;
Add the credit note number.
Click Approve and Save.
You will then see the credit note deducted from the invoice total.
You can check the change has applied correctly by viewing the pending payment within the Middleware or Access PaySuite.