To edit a template with the Template Manager you must first access the Template manager.
You will now see the Template Manager screen.
Note: If you do not see Template Manager listed as an option on the Extensions menu, then it is most likely because you are not logged in as a Super Administrator. Only Super Administrators will see this menu item.
Styles will be highlighted, click on Templates which will turn the view to Template Manager:Templates.
Select the template you wish to modify which will change the view to Template Manager: Customise Template view.
The available files for editing vary from template design or what the template designer allows for editing.
One common change is to use your own graphic/image. Graphics are linked to the HTML file. Simply change the reference to the image of your choice. Keep in mind that if it is a different size than the original image this may change the appearance of the site in unexpected ways.