After upgrade, javascript and CSS are cached from old site

The first time I viewed my shop after upgrading from 1.3.x to 2.0.10 it looked screwed up.

Hitting function key F5 reloaded everything, which is fine. But my existing customers may not know how to do this.

How can I force the javascript and CSS to refresh the first time an existing customer comes back to my site?

You can’t flush the cache in the browser from the server. If the modification time of a file that is named the same in both versions (like styles.css) is newer in the old version than the new version then your users will have to flush their browser cache. If you have shell access do a

touch /styles.css

The browser should now see that the new css file is more recent than what is cached and load it.