1. To change the .Net version of your website, login with remote desktop and open Internet Information Services Manager on the windows 2008 server. This is found under Start Menu > Control Panel > Administrative Tools > Internet Information Services (IIS) Manager.

2. Click on the + next to your server name to expand and then click on Application Pools.

3. Locate the application pool for the website you are changing asp.net for. Right click on the application pool and click on Basic Settings.

4. From the drop down menu under .NET Framework Version choose the version of asp.net you wish the site to run on. If asp.net version you wish to use does not appear on the list, you need to install that .net version first. Just Google the .net download, you would find the link at Microsoft for download.

From Webcosmo Webmaster Forum http://www.webcosmoforums.com/windows-server/21310-how-change-asp-net-version-iis-7-a.html