to change the Application Pool for a web site in IIS...

...select the web site in IIS and then click on "Advanced Settings..." ...The application pool will, amongst other things, dictate what version of the .NET framework a site runs within

older App Pool notes:

1. http://tom-jaeschke.blogspot.com/2012/06/iis7-notes.html
2. http://tom-jaeschke.blogspot.com/2012/12/change-framework-version-of-aspnet-for.html