Netfx3 in Windows 8 or 8.1
Installing the .NET Framework 3.5 on Windows 8 or 8.1
if you install an app that requires a specific version of the .NET Framework, the app's setup program might install the .NET Framework on your computer or prompt you to install it. For example, the .NET Framework 3.5 is not automatically installed with Windows 8 or Windows 8.1. To run apps that require the .NET Framework 3.5 on Windows 8 or later, you must enable version 3.5 on your computer. There are two ways you can do this: by installing or running an app that requires the .NET Framework 3.5 (that is, by installing the .NET Framework 3.5 on demand), or by enabling the .NET Framework 3.5 in Control Panel. Both options require an Internet connection.
In general, you should not uninstall any versions of the .NET Framework from your computer, because an app you use may depend on a specific version and may break if that version is removed. Multiple versions of the .NET Framework can be loaded on a single computer at the same time. This means that you do not have to uninstall previous versions in order to install a later version.