Hello naomi-freedom2se,
Welcome to the Community - thank you for your question!
You will need to edit the master template. To do this, you will need to go to the cog wheel and then 'custom form styles'. Once there, edit whichever template is marked as Master. You can then upload your logo. You cannot change the standard email template that sends with a credit note - you will need to edit this every time.
Thanks,
Talia