Prerequistes:
- 
download codeigniter 3.1.2 (my version 3.0.6) from https://www.codeigniter.com/download
 
- 
download web template for example Frequent Flyer a Travel Category Flat Bootstrap Responsive Web Template from https://w3layouts.com/frequent-flyer-travel-category-flat-bootstrap-responsive-web-template/
 
After you extract codeigniter 3.1.2 you may have these structure directory:
[root codeigniter]applicationsystemuserguide
To upgrade codeigniter 3.0.6 to 3.1.2, extract codeigniter 3.1.2 and 
copy/replace system folder into [root codeigniter] folder. 
Extract your template into codeigniter folder [root codeigniter]. 
[root codeigniter]applicationsystemuserguidefft--css--fonts--images--js--video--index.html--w3layouts-License.txt
Create file test.php file in folder controller.
<?phpdefined('BASEPATH') OR exit('No direct script access allowed');class Test extends CI_Controller {  public function __construct() {    parent::__construct();  }  public function view($page = 'Test')  {    $this->load->helper('html');    $this->load->helper('url');    if ( ! file_exists(APPPATH.'views/fft/'.$page.'.php'))
    {
      show_404();
    }
    $data['title'] = $page; // Capitalize the first letter    $this->load->view('fft/templates/pageheader',$data);
    $this->load->view('fft/'.$page, $data);
    $this->load->view('fft/templates/pagefooter');
  }
 }?>
At folder views, create folder fft.
application
--------bodymenu.php--------pageheader.php------test.php
We split file views->test.php. The other supporting files we put 
them into folder  [root codeigniter]->application->templates.
<?phpdefined('BASEPATH') OR exit('No direct script access allowed');?><!-- banner -->  <div data-vide-bg="<?php echo base_url(); ?>fft/video/sea">    <div class="center-container">      <div class="container">        <!-- load body header -->        <?php $this->load->view('fft/templates/bodyheader'); ?>        <div class="logo animated wow zoomIn" data-wow-delay="900ms"> 
         <h1><a href="/web/joomla/index.html">Frequent 
<span>flyer</span><i>take me 
anywhere</i></a></h1>        </div>        <div class="start animated wow bounce" data-wow-delay="700ms">          <a href="#about" class="hvr-bounce-to-bottom scroll">Get Started</a>        </div>      </div>    </div>  </div>
Final directory will like this:
[root codeigniter]
------views
------templates
--------bodyheader.php
--------pagefooter.php------about.php------test.php
--system
--userguide
--fft
----css
----fonts
----images
----js
----video
----index.html
----w3layouts-License.txtReferences:
- https://www.codeigniter.com/download 
- https://w3layouts.com/frequent-flyer-travel-category-flat-bootstrap-responsive-web-template/
- http://www.codeigniter.com/user_guide/installation/upgrade_310.html