Dot walk and update doesn't work, you have to GlideRecord to parent record and update it.
(function executeRule(current, previous /*null when async*/ ) {
var servicetype = current.getValue('u_service_type');
if (JSUtil.notNil(current.parent)){
var gr = new GlideREcord('<parent table>');/add the table name here
gr.addQuery('sys_id',current.parent )
gr.query();
if(gr.next())
{
gr.u_service_type = servicetype;
gr.update();
}
})(current, previous);
The condition of the BR should be that current.parent is not empty AND current.u_service_type Changes
-Anurag
