Javascript for Changing BPF Stages

Hello Guys,

I have been met with a customer requirement that, there is a optionset field in BPF over case entity which have two options 1) Approved 2)Reject. Now they want if it gets rejected then BPF should go to previous stage. And for this I digged many things and now I am landed on a JS which is not working for me.

function ChangeStg()
{
    var approval = Xrm.Page.getAttribute("new_approval").getValue();
    if(approval == 2)
    {
      Xrm.Page.data.process.movePrevious(function (result)
      {
            alert(result);
      });
    }
}

Can you please help me with some working code if you guys have any.