To install Microsoft .NET Framework version 3.5.1 follow the steps below based on your version of Windows.
Windows 8, 8.1, 10. Follow the steps under "Enabling the .NET Framework 3.5 in Control Panel" in this article.
Windows 7. Follow How to install .NET Framework 3.5.1 on Windows 7.
Windows XP and Vista. Download and install .NET Framework from Microsoft Download Center