You can't. You're going to have to learn ASP.NET and re-write the site. ASP and ASP.NET are two completely different beasts.
Dear it's a big job because there is no direct convertor available for asp to asp.net you need to code for that ... It's easy .. http://omexinfotech.blogspot.com
you need to rewrite it anyway to take advantage of great features like master pages. Bit of a bind but worth doing if you have a quality site.