How can I change the title of the Home link in the navigation bar?

wp_nav_menu with no arguments and no menus defined in the back end at wp-admin->Appearance->Menus (which sounds like the circumstance you describe) just lists your pages, plus a “Home” link.

That is done by calling wp_page_menu, the default callback for wp_nav_menu, which means that under those conditions you can alter that text by hooking to wp_page_menu_args and setting, or changing, the show_home argument.

function alter_default_home_link_wpse_1011444($args) {
  $args['show_home'] = 'Home is where the heart is';
  return $args;
}
add_filter( 'wp_page_menu_args', 'alter_default_home_link_wpse_1011444');