How to change the series of credit note number

You may need a 3rd party app as a workaround.

Good to see you here in the Community, akashxdesign.

Changing the series of the credit note number is currently unavailable in QuickBooks Online. Credit Notes use the same number series as invoices.

I'd suggest submitting feedback within your QuickBooks Online (QBO) account. This way, it goes to our product developers to help improve your experience while using the program. Here's how:

1. Go to the Gear icon at the upper right and select Feedback under Profile.
2. Enter your comments or product suggestions. You can also attach a screenshot or file to combine them all. 
3. Click Next.
   
4. You'll be provided a list of suggested help articles related to your comments. You can read through them or choose Skip and send message.
5. Select the appropriate category from the drop-down menu.
6. Tap Send message.

For now, you can visit our QuickBooks Blog site for product updates as well as business tips from other accountants. Also, you'll want to consider visiting this link: Create and apply credit notes or delayed credits in QuickBooks Online. This will provide you with steps on how to apply delated credit.

I'm only a post away if you need more help in completing your other tasks in QuickBooks. It's always my pleasure to help you out again.