Here is the steps to migrate opencart website from one server to another server.

1. Download ALL the files from old server.
2. Take a backup of Database using PhpMyAdmin.
3. Upload files to NEW SERVER.
4. Install Database on NEW SERVER
5. Update the configuration files i.e config.php and admin/config.php with new domain path.

Update config file with following details.

<pre>
// HTTP
 
define('HTTP_SERVER', 'http://yournewdomain.com/');
 
// HTTPS
 
define('HTTPS_SERVER', 'http://yournewdomain.com/');
 
// DIR
 
define('DIR_APPLICATION','/root-folder-newdomain/public_html/catalog/');
define('DIR_SYSTEM','/root-folder-newdomain/public_html/system/');
define('DIR_DATABASE','/root-folder-newdomain/public_html/system/database/');
define('DIR_LANGUAGE','/root-folder-newdomain/public_html/catalog/language/');
define('DIR_TEMPLATE','/root-folder-newdomain/public_html/catalog/view/theme/');
define('DIR_CONFIG','/root-folder-newdomain/public_html/system/config/');
define('DIR_IMAGE','/root-folder-newdomain/public_html/image/');
define('DIR_CACHE','/root-folder-newdomain/public_html/system/cache/');
define('DIR_DOWNLOAD','/root-folder-newdomain/public_html/download/');
define('DIR_LOGS','/root-folder-newdomain/public_html/system/logs/');

// DB
 
define('DB_DRIVER','mysql');
define('DB_HOSTNAME','localhost');
define('DB_USERNAME','your-new-database-username');
define('DB_PASSWORD','your-new-database-password');
define('DB_DATABASE','your-new-database');
define('DB_PREFIX','oc_');

</pre>

Save the changes and upload files on new server.