if you have included jQuery it should be as easy as:
// first, give your select box an ID
<select id="myselect">
...
</select>
// add the following onChange script
<script>
$('#myselect').change(function() {
var value = $(this).val();
if(!value) {
// empty value and make the input editable
$("#ver-city").val("").prop('readonly', false);
return;
}
// set the value and make sure the user cannot edit it
var name = $("option:selected", this).text();
$("#ver-city").val(name).prop('readonly', true);
});
</script>