How to reset select box after "selected" has been assign

How to reset select box after "selected" has been assign

Hello,

Im trying to reset("selectedItem",0) on a dropbox that has had a "selected" attribute assigned to it.When I try to do the "selectedItem,0" property it does not reset the box.
 
How do I do this?

$("#s1tpid").prop('selectedIndex',0);
$("#s1elem").prop('selectedIndex',0);