how to refresh Select2 dropdown menu after ajax loading different content?

Select 3.*

Please see Update select2 data without rebuilding the control as this may be a duplicate. Another way is to destroy and then recreate the select2 element.

$("#dropdown").select2("destroy");

$("#dropdown").select2();

If you are having problems with resetting the state/region on country change try clearing the current value with

$("#dropdown").select2("val", "");

You can view the documentation here http://ivaynberg.github.io/select2/ that outlines nearly/all features. Select2 supports events such as change that can be used to update the subsequent dropdowns.

$("#dropdown").on("change", function(e) {});

Select 4.* Update

You can now update the data/list without rebuilding the control using:

fooBarDropdown.select2({
    data: fromAccountData
});