How do I change a field description using hook_form_alter()?
I found a solution. 

/**
 * Implements hook_form_alter().
 */
function mymodule_form_alter(&amp;$form, FormStateInterface $form_state, $form_id) {
  if ($form_id == 'my_form_id') {
    $form['field_myfield']['widget'][0]['value']['#description'] = \Drupal\Core\Field\FieldFilteredMarkup::create(t('My custom description'));
  }
}