You can create a credit note by duplicating the invoice and adding a negative symbol "-" to the item amounts.
Here’s how:
Go to the Invoices section.
Select the invoice for which you want to create a credit note
Swipe up on the lower menu and select Duplicate
A new invoice will be created with all the information from the original invoice
Change the amount of each item to a negative amount by clicking on the item and adding the "-" symbol in front of the price
Press Save for each item
Press Done to save the negative invoice/credit note
Note: The credit note will appear among the other invoices in the invoice list