I got the solution,

Instead looking for client side solution we can do it via server scripting via GlideRecord.

current.update is updating all the fields. so using below code we can get the solution.

var gr = new GlideRecord('incident');
gr.addQuery('sys_id',current.sys_id);
gr.query();
while(gr.next()) {
gr.description = current.description;
gr.update();
} 
action.setRedirectURL(current);