First of course you have to sign in, then Select Layout ->> Edit HTML. there are postslike this "before editing your template, you may want to save a copy of it." Download FullTemplate>, click on Download Full Template and save the file, usually an xml file format.
if you want to install a new template or templates that have been edited or you want to restore the template as before, you can click on the Upload button below.
may be useful,, ^ ^