While there isn't a direct rename option, there is a workaround to rename your custom report templates. Go to Settings > Billing & Invoices > Invoice Templates and select Custom from the Show drop-down. Choose the template you want to rename and click Duplicate from its row-action menu. Enter the new name in the Duplicate Template dialog and click Duplicate. Now that you have duplicated the template under a new name, you can delete the old one.
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