Uninstalling themes is a simple process, essentially the reverse of installing. First go to the Theme Manager and make sure that the theme you wish to install is not currently enabled. Once you have verified that it is disabled, then access your server. On the server, find the directory containing the theme files and delete the files and the directory. That's all there is to it!
Note that Drupal is very forgiving, and erroneous deletion of an active theme will not crash your site, it will simply result in the content being shown without any styling.