1. Backup your existing all files and database.

2. Replace all files 3.0 version files with new 6.2 version files

3. Update your database connection as per your database 
    at aplication->config->database.php

4.  Update following in your config file 
    at application/config/config.php file 
    $config['installed'] = FALSE; to $config['installed'] = TRUE;

5. if your domain ssl certified then pls add https instead of http in the 
     application->config->config.php
    $root = "http://".$_SERVER['HTTP_HOST']; to $root = "https://".$_SERVER['HTTP_HOST'];

6. Run first update_1.sql , update_2.sql , update_3.sql and  update_4.sql then update_lang.sql in you existing database from 
    phpmyadmin

7. Replace upload directory with your existing directory one by one inside
    assets/uploads/

  Note: That way your old directory will be there with uploaded resource and new directory also remain
       Because in your directory there are some uploaded resources exists and we have newly created some directory.
       so, all directory should be there.

Thank you! 

