Credit Note is generated from a Customer Invoice that has been posted to financial system.
Navigation → Documents → Processing → Customer Invoices → Select Invoice from List and Double Click to Open.
1. Select the Invoice Details tab.
2. Specify Qty to Credit.
3. On top tool bar select the Generate Credit Note from Invoice 
icon or click Processing on top menu and select Generate Credit Note from Invoice.
4. Save using the save 
icon or CTRL + S or CTRL + Enter to save and close.