please ftp into your site
after you ftp into your site, navigate into your wp-content dir
after you navigate into your wp-content dir, navigate into your themes dir
after you navigate into your themes dir, navigate into you thesis install dir--my thesis install dir is labled as thesis_182 (not sure if it's static)
after you navigate into your thesis install dir, delete the folder labeled either custom or custom-sample
after you have deleted custom/custom-sample (or both), upload the folder labeled "custom" from the skin you downloaded
sidenotes: sometimes, the skins come with an options data file. this file is part of the theme switching process, but not the theme itself--you don't have to upload this file onto your site. Please log into your wordpress admin panel and navigate to thesis -> manage options. upload the options data file onto the appropriate choice--it should say if it's a site/design/all options file.