How do i change date format in View Exposed Date filter?
To make it easy let's disable Drupal datepicker and make it with javascript.

First disable jquery ui datepicker from exposed filter (like picture below)


Then create a module with libraries in file my_module.libraries.yml like below

d8_datepicker:
  version: 1.x
  js:
    https://code.jquery.com/ui/1.12.0/jquery-ui.min.js: { type: external, minified: true }
    js/datepicker.js: {}


And in our my_module.module let's alter the exposed form and add line below :

  function my_module_form_alter(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id){
     if ($form_id == 'views_exposed_form') {
      $form['field_date_value']['#type'] = 'textfield';
      $form['field_date_value']['#attributes']['class'][] = 'datepicker-text';
      $form['field_date_value']['#attributes']['placeholder'] = 'yyyy-dd-mm';
      $form['#attached']['library'][] = 'd8/d8_datepicker';
      $form['#validate'][] = 'my_exposed_validate';
     }


In our validate form function we will change date from our custom format to format needed by drupal :

function my_exposed_validate(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state) {
  if(!empty($form['field_date_value']['#value'])) {
    $dt = date_create_from_format('Y-d-m', $form['field_date_value']['#value']);
    if($dt!==FALSE) {
      $value = $dt-&gt;format("Y-m-d");
      $form_state-&gt;setValue('field_date_value', $value);
    }
  }
}


And finaly in our file datepicker.js add this line : 

(function($) {
    $('.datepicker-text').datepicker({ dateFormat: 'yy-dd-mm' });
})(jQuery);


Hope this helpful and if you wan't to go further, alter a form validation to check if the format sended is date or not ;)