where to change text for header?

You can use twentyfifteen_custom_header_args filter for customizing width and height. Check following example.

add_filter( 'twentyfifteen_custom_header_args', 'theme_slug_custom_header_args' );
function theme_slug_custom_header_args( $args ){
  $args['width']  = 600;
  $args['height'] = 800;
  return $args;
}

In the example, width is changed to 600 and height is set to 800.