May be too late to answer, but at least some one will get help.
You can try two options:
This is the result when you want to assign based on index value, where '0' is Index.
$('#mySelect').prop('selectedIndex', 0);
don't use 'attr' since it is deprecated with latest jquery.
When you want to select based on option value then choose this :
$('#mySelect').val('fg');
where 'fg' is the option value