How can I copy a field to another field?

I have an Article content type for which I want to change the machine name of its fields. I created new fields with a different machine name.

How do I copy the old fields to the new fields?

You can run this, in hook_update_N():

$storage = \Drupal::entityTypeManager()->getStorage('node');
// Load all the nodes IDs for nodes that don't have the new value set.
$nids = $storage->getQuery()
  // Load all nodes for which the new field has no value.
  ->notExists('new_field_name')
  ->execute();

// Loop through the nodes.
foreach ($nids as $nid) {
  // Load the current node.
  $node = $storage->load($nid);
  // Set the new field name to the old field name and save.
  $node->set('new_field_name', $node->get('old_field_name')->value)
    ->save();
}