You can use dispatchEvent or fireEvent as required.

Check Link for further details.

Update

Check the below snippet for working sample

document.getElementById("MyDiv").addEventListener('change', function (event) {
    alert(event)
    var elem = event.target;
});

document.getElementById("myInput").addEventListener('change', function (event) {
    alert(event)
    var elem = event.target;
});


function fnc() {
  document.getElementById("myInput").value = "New";
  var event = new Event('change');
    document.getElementById("myInput").dispatchEvent(event);
  document.getElementById("MyDiv").value = "Somethinig";
}

<div id="MyDiv">
  hi
</div>

<input type="text" id="myInput" placeholder="Text" />

<button onclick="fnc()">Click to fire event</button>

ok, i solved with

function test_1()
{
    document.getElementById("MyDiv").dispatchEvent(new Event("change"));
}

as suggest by "Sebastian Simon" thank you!

so, to be complete, the problem was about apply Select2 on the dropdown. Select2 doesn't fire native Onchange, but fires its. that mean "addEventListener" will not fired when your dropdown change value. to do that, i created on its event a textbox, appended to him, fired the Onchange texbox and than remove it.

for example:

            $('.js-example-basic-single').select2({dropdownAutoWidth : true});
            $('.js-example-basic-single').on('change', function (e)
            {
                tmp_div = document.createElement("input");
                e.target.appendChild(tmp_div)
                tmp_div.dispatchEvent(new Event("change"))
                e.target.removeChild(tmp_div)
            });
<div>
  <input value="" id="MyElement" type="text">
  <select id="cmb_UM1" class="js-example-basic-single select2-hidden-accessible" data-select2-id="select2-data-ctl00_Content_cmb_UM1">
    <option selected="selected" value="" title="" data-select2-id="select2-data-2-vplr"></option>
    <option value=" 1001171997" title=" 1001171997" data-select2-id="select2-data-12-gcgl"> 1001171997</option>
  </select>
</div>

You can also try calling back the first function(used for alert) when entering values with different buttons .

Or you can create another alert when button is clicked to enter values

document.getElementById("MyElement").addEventListener('change', test_2)

function test_2() {
  alert(event)
  var elem = event.target;
};

document.getElementById("MyDiv").addEventListener('click', test_1)

function test_1() {
  tmp = document.getElementById("MyElement");
  tmp.value = "new value";
  test_2();
}

<div>
  <input value="" id="MyElement" type="text">
  <button id="MyDiv">Try me</button>
</div>