Question: I have an application developed using the 2.0 framework, then I need to migrate it into 3.5 framework? is there any fastest way and best approach to do this?

Answer: 
There is no migration. .NET 3.5 is .NET 2.0 SP2 plus some new assemblies.
I think you have to use the VS IDE to change the .NET edtion,if you change the .net edition,it may cause some bug,so,after you changed it,you'd better rebuild the solution at first.