How do I update all nodes?
This is just a suggestion, its not well designed solution.

Considering that you already added your field to a Node type and you need update its value...

You can create a function that loads all nodes with specific type:

$articles = \Drupal::entityManager()-&gt;getStorage('node')-&gt;loadByProperties(array('type' =&gt; 'article'));;


And use a foreach to update your field content:

foreach ($articles as $nid =&gt; $article) {
  //Change what you need
  //Save your changes
}


Now, just run this once, on a new module install function or whatever you want.