To create or update a credit note number sequence go to the apps section on the top right corner of the screen & select the Expense management option (Click here to upgrade your package to enable Expense management.).
Once the Expense management dashboard opens click on the Credit Debit Note option, Click on Create then the Credit Note option
Create a credit note page should open up. On the left top corner, a Credit note number column should be visible, click on the settings icon right beside it, a new tab will open up, Select whether you would like to continue to Auto-generate the Credit note number with a prefix of your choice or add the same manually each time. Click on Save and save the changes made.
Click here to learn how to assign a debit note number to your debit notes.