To migrate your existing Entity Framework 4.x project to Entity Framework 5.0 using VS2012, first target .NET Framework 4.5:
Second, remove the existing Entity Framework 4.1/4.3 reference and install Entity Framework 5.0 from NuGet:
Search for Entity Framework online and install it. Make sure that the version is 5.0:
If you check the version of the Entity Framework reference, then it should be 5.0.0.0:
If you open your existing data model (.edmx) file then it may give an error, as it is still using EDMX version 2.0 (open edmx as XML). Click on the 'Modify' link on the designer and it will be modified to version 3.0.
Now your existing Entity Framework project will work in Entity Framework 5.0 without any major changes.