altering a code – how?

Hi folks,

I have encountered a problem with AdSense plugin, which disabled posting comments when running. I have a communication with their team and finally they advised me this:
“There must be logs on the server or in your site’s root directory – error_log file, it is necessary to check it. Besides, in your wp-config.php file in your site’s root directory you can change

define(‘WP_DEBUG’, false);
for
define(‘WP_DEBUG’, true);
and check if there are any errors on the screen.”

While I do not exactly know what to do in the root directory thing, I managed to find that php file in the root directory, but I cannot alter it from ‘false’ to ‘true’. Can you advise me how to alter that file according to their instructions?