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.
Open your project in Visual Studio (keep a backup) then goto project properties and set the target .net framework to 3.5.

save your settings and then compile the project, if you find any errors resolve them and you are good to go.