How do I change .css versioning?

In using the wp_enqueue_style you can set a version number similar to how you set the script.

Codex Example:

wp_enqueue_style( string $handle, string $src = false, array $deps = array(), string|bool|null $ver = false, string $media="all" )

Your Code

Say you want to update the ‘conferences’ CSS from above.

wp_enqueue_style( 'conferences', get_template_directory_uri() . '/css/cars.css', array(), '1.3.1' );