Why are changes to my HTML message not reflected in the preview window?

The preview window updates in real time as you edit your message. However, if your message is HTML and it references external resources on a web server (like image files or CSS files), and you make changes to those files (like editing a CSS file stored on your web server), Direct Mail may not always be able to detect those changes (at least for purposes of importing or previewing your message). If you have made changes to documents on your web server, you may need delete the Direct Mail's cache file saved on your computer. Here how:?