When enqueueing a script or style, use `filemtime` to set the file version to the Unix timestamp for when the file was last modified

functions.php

<?php

/* Obviously, add this to a function that hooks into wp_enqueue_scripts() (or admin_enqueue_scripts()) */ 

wp_enqueue_style( 'main', get_stylesheet_directory_uri() . '/style.css', false, filemtime( get_stylesheet_directory() . '/style.css' ) );