How to update site styles?

Question

Node.js

The question of such a plan, you need to make minor css changes to the theme on Sage (Wordpress)
scss are located at the address if you look in the
webpack inspector: ///./styles/common/_global.scss
Well, the root folder with the template is package.json
and update.php with content

$cmd = 'yarn build';
exec( 'cd ' . __DIR__ . ' && ' . $cmd, $out, $res);
if ($res === 0) echo '<h1>OK!</h1>';
else echo '<h1>Fail!</h1>';

As I understand it, you need to make changes to scss and then either run
yarn build through the console
or run this php file? what is equivalent?

in fact, it is necessary to make 2 lines of amendment, but some kind of trouble.