How to change the .NET Framework version

The .NET Framework can be changed in Control Panel by clicking the ASP .NET Version icon in the Site Tools section.

There are two options:
.NET 4.x
.NET 2.0/3.0/3.5 sp1
By default the version is set to 2/3/3.5, which encompasses all previous .NET Framework versions, with the exception of 1.1, which we do not support.

You can only have one .NET Framework setting for your site. You cannot have both 4.0 and the previous versions running on your site simultaneously.