Create a Credit Memo Template and a Credit Memo Email Template. Add these templates on Account or Account Location.
Go to Setup > Custom Settings > BillingSystem properties and check the Generate Credit Memo Documents to generate Credit Memos in DOC, DOCX, PDF or RTF format. Refer to Billing System Properties for other key settings.
Go to View Object from the Credit Memo object. From Buttons, Links, and Actions, edit New label. Add a VF page ‘SetAccountDefaultCMTemplate’ to set the default Credit memo template.