From your Home page, navigate to the "Templates" section.

Click the "Reports" tab.

Click any template to open it for editing.

Apply your desired changes within the report editor, saving regularly. When you're done, save one final time, then to exit, click the X in the upper right hand corner.

Note: Your changes will only affect the template, and not any reports previously created from this template. To overwrite a report with a new template, see here.