Hi Kanaga Kumar,
Try the below code:
app.findGrepPreferences = app.changeGrepPreferences = null;
app.findGrepPreferences.bulletsAndNumberingListType = ListType.BULLET_LIST;
app.changeGrepPreferences.appliedParagraphStyle = "CN";
app.activeDocument.changeGrep();
alert("Process Completed")
Regards
Siraj