Asp.Net MVC Implementing generic IRepository for different ORM and ADO.NET

I implemented this solution : https://chsakell.com/2015/02/15/asp-net-mvc-solution-architecture-best-practices/

I would like to discuss with you about Data Access Layer. In that architecture Repository Pattern implements Entity Framework. I don't understand which classes I have to modify so that Repository implements different ORM like NHibernate or different DBMS connection (oracle, postgres...etc).