Answers for "how to set an element up the select2"

select2 clear options

Remove the selected options : 
-----------------------------
$('#mySelect2').val(null).trigger('change');

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Completly remove the select2 initialization :
--------------------------------------------
$('#payment_method').html('').select2({data: [{id: '', text: ''}]});

select2 replace options

var initQuantitiesDropdown = function () {
    var options = [];
    var selectedFruit = $("#fruits").val();
    $.each(pageData.products[selectedFruit].quantities, function (key, value) {
        options.push({
            text: value,
            id: key
        });
    })
    $("#quantities").empty().select2({
        data: options
    });
};

$("#fruits").select2().change(initQuantitiesDropdown);
initQuantitiesDropdown();