This is pretty easy : Just Listen on the change event on SELECT and then change the value of other drop down as below.
$(document).ready(function(){
$("#dropdown1").on("change",function(){
var other= $("#dropdown2");
var value=$(this).val();
switch (value) {
case "1": other.val("6"); console.log('hi1',other); break;
case "2" : other.val("7");break;
case "3" : break;
case "4" : break;
default : console.log('hi');;
}