Basic style.css query

I find I need to edit style.min.css to make changes to my site. Changes to the style.css have no effect. This is pretty arduous. How can I make changes in the more readable plain style.css and have them reflected in my site?

I guess I need to change the css file the site is calling or set up some sort of minify plugin to re-write my style.min.css from the style.css. I don’t know how to do either.

Any help much appreciated.