Opencart 3 

Open /system/config/default.php file and change the param called 'session_engine'.
// Session
$_['session_engine']       = 'db';
Possible values: 
  • db – sessions are stored in the database, oc_session table;
  • file – sessions are stored in a directory specified in DIR_SESSION constant in the config.php.

Opencart 2.3

Open /system/framework.php and find this line:
$session = new Session();
And modify it:
$session = new Session('file');
Possible values: 
  • db – sessions are stored in the database, oc_session table. Actually doesn't work, because corresponding driver wasn't finished and requires further development; 
  • file – session are stored in a directory specified in session.save_path param in PHP configuration;
  • native – session are stored in a directory specified in session.save_path param in PHP configuration.