It is not possible to change the targeted .NET framework for a mixed-mode C++/CLI project. In the Common Properties > Framework and References page there is a static control that shows the target version, which is by default v4.0. It is not possible to change this from this page or other page. The only way to do it is manually changing or adding if missing this entry in the .vcxproj file:
Since multi-targeting for the .NET framework is possible, it should be possible to change it even for mixed-mode projects, otherwise the DLLs version is picked up automatically to version 4.0.
This was possible in Visual Studio 2008!