The left navigation menus are edited on the on the Menus screen.
To edit left navigation menus, select a the menu from the drop-down that corresponds to the section being edited. Once the page reloads, the interface can be used to search for links on the left side of the screen and added to the right side. Once the links are added to the right side, they can be rearranged and indented to create hierarchy.
Once your changes are complete, click Save Menu to publish your changes.