Hi,
From the download page for the "Microsoft .NET Framework 4.6",
The Microsoft .NET Framework 4.6 is a highly compatible, in-place update to the Microsoft .NET Framework 4, Microsoft .NET Framework 4.5, Microsoft .NET Framework 4.5.1 and Microsoft .NET Framework 4.5.2.
On other words, having .NET 4.6 installed (it's part of Windows 10 so you need not manually install it) means you have .NET 4.0, 4.5, 4.5.1, 4.5.2 and 4.6 installed and can run code targeting any of those framework versions without making any modifications
to your Windows installation.
You cannot install .NET 4.0 when you have a later version installed as each version is an in-place update to the last.
Either you could install Visual Studio 2015 as the Integrated Development Environment.
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