Migration from Asp.Net MVC 2.0 to 5.0

With each release, it seems there are things that were added, changed or removed. The cleanest approach would be to start an MVC5 project and start moving your code into this new project (this is what I would do - unless of course your project is really massive).

Depending on the complexity of your project, you might find it better to upgrade one version up each time and using provided upgrade tools (such as this one http://blogs.msdn.com/b/marcinon/archive/2011/01/13/mvc-3-project-upgrade-tool.aspx for moving from 2 to 3). Or manually upgrading by going through the upgrade guidelines with each framework version (example get the document
ASP.NET-MVC-3-Tools-Update-RTM-Release-Notes.doc from http://www.microsoft.com/en-us/download/details.aspx?id=1491)

I completely agree with Mr. Praveen create a new empty MVC 5 project, and move all your files there from the old MVC 2 project, but firstly I suggest you to create a backup and don't try to change your .dll file. There is a lot of difference between MVC 5 and MVC 2 like .net framework and features. If you want to update your .dll file then it's very difficult.