WP-config update for multisite

Hi, checked around and didn't see this specific question so figured I'd ask away! When updating the wp-config file with code snippet below, are we supposed to save that to the main root of our website or should it be saved to the child theme so it's not overwritten when WP has an update? I'm going to do it to the main root at this time, but just wondering if it can be placed in both or if it should be in one over the other due to the updates that occur on WP. Thanks!

/* Multisite */
define('WP_ALLOW_MULTISITE', true);