Hi,
It depends, if you are using Express Accounts there is no Prefix case option, if you are using Express Invoice the Prefix case option is within Options(preferences if you have a Mac) > Credit Note > Credit note numbering > Prefix(optional).
None of the above will allow to change existing credit notes numbers, only Express Invoice allows to change the sequence of the credit notes for a new number, within the same menu, but not for previous credit records.
For more information I suggest to contact NCH support > supportnch@nchsoftware.com
Best regards,