It's as easy as in your Visual studio.
Project Properties -> Compiler Tab -> Advanced Compile Options button
Project Properties -> Application Tab
With Respect to .NET Framework 4.6 and Visual Studio 2017 you can take the below steps:
It depends which version of Visual Studio:
Newer versions of Visual Studio support many versions of the .Net framework; check your project type and properties.
You can also search the Visual Studio project files for the XML tag RequiredTargetFramework. This tag seems to exist on .NET 3.5 and higher.
For example: <RequiredTargetFramework>3.5</RequiredTargetFramework>
Simple Right Click and go to Properties Option of any project on your Existing application and see the Application option on Left menu and then click on Application option see target Framework to see current Framework version .
The simplest way to find the framework version of the current .NET project is:
You can't change the targeted version of either Windows or the .NET Framework if you create your project in Visual Studio 2013. That option is not available anymore.
Look that link from Microsoft: http://msdn.microsoft.com/en-us/library/bb398202.aspx
Source: Stackoverflow.com