You can open the Properties file in the Solution.
In that new page, you can look for Application tab in that tab there are settings for your project. There you can set the target framework for your application, all of the installed Frameworks would be listed there, choose from them.
For my project, this was the setting: http://s21.postimg.org/p2yt03snb/Screenshot_102.png
] you can follow these as general example and edit yours.
: .NET Framework 4.5 must be
installed on your system in order to run and use that, otherwise there is no other option for you to test your application's behaviour in 4.5th version.