You can only edit report templates belonging to you or the organisation to which you belong (assuming you have the necessary admin privileges).
To edit a report template go to your Report Template Library from the dashboard. The templates that you can edit will have a report template builder link.
To edit the name and tags of a template click on the corresponding edit icon.
To edit the content of a template, click on the report template builder link and you will be directed to the Report Template Builder for that particular template. You can then add / remove sections, and add / edit content within a particular section in the usual way just as you would if you were editing a report within the Report Builder.