How to change the ENVIRONMENT type

When open the index.php file to change the ENVIRONMENT type I have found the following:

PHP Code:

define('ENVIRONMENT', isset($_SERVER['CI_ENV']) ? $_SERVER['CI_ENV'] : 'development'); 


In the previous CI version was as the following:

PHP Code:

define('ENVIRONMENT', 'development'); 


And I could change the environment by replace "development" word to ("production" or "testing").

How can I do that in CI 3 ?