In my game, I want run time modification in object body so I want to modify the joint def according to situation. So provide comment for how to modify jointdef run time.
There is no reason to do that unless you plan to reuse it. JointDef doesn't really do anything, as the name suggests, it is only a definition and even though you can change the fields, the Joint that was created using the JointDef will not change, as you probably discovered.
What you want to do is store a reference to the Joint that is created using the JointDef and then manipulate the Joint itself.
What exactly are you trying to accomplish? Specifying the problem better will help us provide you with a better answer.