The default template set will never be able to be modified in the admin panel, you will be working with another template set.
The reasons for this, and also why you should never edit geo_templates/default/ manually (using FTP or other means):
Instead, make alterations in your own custom template set. When you first view any pages in the Design section2), there will be a tool to create the starter custom template set, which will be a copy of the main files from the default template set. By default the starter template set is named my_templates but you can change the name during creation if you wish.