WordPress CSS doesn’t update

Sounds like your Template has 2 style sheets, one that is unused.

By default WordPress will look for style.css in the editor but this doesn’t necessary mean its used in the header.php

Take a look at your source code and view which css files are being loaded.

I suspect your see it something like: /wp-content/themes/your-theme/css/blah.css

Rather than /wp-content/themes/your-theme/style.css