Windows 8 comes with .NET Framework 4.5 already installed, but in order to run properly, some apps might require an earlier version (such as 3.5 or 2.0). There are two ways to install an earlier version: install it on demand, or turn it on in Control Panel. Both options require that your PC be connected to the Internet.
.NET Framework 3.5 also includes versions 2.0 and 3.0.
.NET Framework 4.5 replaces version 4.0.
You can have more than one version of .NET Framework on your PC at the same time. In general, you shouldn’t uninstall any version of .NET Framework, because it might cause apps that require that version not to work.
If an app needs a specific version of .NET Framework but can’t find it on your PC, it might automatically install and activate that version. If the app prompts you to start the installation, tap or click Install this feature.
Under View choose Large icons, and then tap or click Programs and Features.
Tap or click Turn Windows features on or off.
You might be asked for an admin password or to confirm your choice.
Select .NET Framework 3.5 (includes .NET 2.0 and 3.0), and then tap or click OK.
More info on installing .NET Framework 3.5 on Windows 8