Credit Notes follow the same numbering sequence as Invoices.
Click on Cog Symbol (top right) > Accounts & Settings > Sales
Now turn on Custom Transaction Numbers.
This will allow you to amend Credit Note & Invoice numbering.
Be careful ... if you add a C behind your credit note numbers, the next time you open an invoice it will have a C behind the invoice number which will need to be deleted.
Credit Notes follow the same numbering sequence as Invoices.
Click on Cog Symbol (top right) > Accounts & Settings > Sales
Now turn on Custom Transaction Numbers.
This will allow you to amend Credit Note & Invoice numbering.
Be careful ... if you add a C behind your credit note numbers, the next time you open an invoice it will have a C behind the invoice number which will need to be deleted.