ASP.NET MVC is a development framework for building web applications using the MVC (Model, View, Controller) design. Kentico 8.x comes with a specific version of ASP.NET MVC included in the installation.
If you use the CMSApp_MVC project (obsolete approach) for your MVC development and want to upgrade ASP.NET MVC to a newer version, you will also need to upgrade the Kentico MVC library to the same version.
This approach package doesn’t apply to MVC development using an external application (recommended for Kentico 9).
Please read this document before upgrading ASP.NET MVC.