Drupal - How to use presave hook to save a field value as node title?

You need to implement the hook_entity_presave()

/**
 * Implements hook_entity_presave().
 */
function YOUR_MODULE_entity_presave(Drupal\Core\Entity\EntityInterface $entity) {
  switch ($entity->bundle()) {
    // Here you modify only your day content type
    case 'day':
      // Setting the title with the value of field_date.
      $entity->setTitle($entity->get('field_date')->value);
     break;
  }
}

For entity of type user

/**
 * Implements hook_entity_presave().
 */
function YOUR_MODULE_entity_presave(Drupal\Core\Entity\EntityInterface $entity) {
  $entity->field_uhid->value = 'testing';     //set value for field
}

For entity of type profile i have used below code

/**
 * Implements hook_entity_presave().
 */
function YOUR_MODULE_entity_presave(Drupal\Core\Entity\EntityInterface $entity) {
  if ($entity->getEntityType()->id() == 'profile') {
    $zipcode = $entity->field_zip_code->value;
    $url = "http://maps.googleapis.com/maps/api/geocode/json?address=".$zipcode."&sensor=false";
    $details=file_get_contents($url);
    $result = json_decode($details,true);
    $lat=$result['results'][0]['geometry']['location']['lat'];
    $lng=$result['results'][0]['geometry']['location']['lng'];
    $entity->field_geolocation->lat = $lat;
    $entity->field_geolocation->lng = $lng;
 }
}