Change Dropdown to checkbox in View
Not sure where you're trying to do this, but if it's a node edit form, you can just change the widget to "Check boxes/radio buttons" on Manage Form Display (admin/structure/types/manage/&lt;content_type&gt;/form-display). Make sure the Options module is installed.

For an exposed filter in a View, say, you can use a hook_form_alter to change the #type to whatever you want; e.g.:

function YOUR_MODULE_form_views_exposed_form_alter(&amp;$form, FormStateInterface $form_state) {
  $form['field_your_field']['#type'] = 'checkboxes';

  // If there's no user input (code not shown), rebuild the form.
  $form_state-&gt;setRebuild();
}


Only makes sense if #multiple is set to TRUE of course.

UPDATE: Added the call to setRebuild().