Inserting in user's field on creating the user?
If you're using Drupal 8,
See This one.

Update a node:

Example 1 : Update title

$node = \Drupal\node\Entity\Node::load($nid);
$node-&gt;setTitle('The new Title')
$node-&gt;save();


Example 2 : Update a field ('body' and 'field_name')

$node = \Drupal\node\Entity\Node::load($nid);
$node-&gt;set("body", 'New body text');
$node-&gt;set("field_name", 'New value');
$node-&gt;save();


To make changes after click on save / edit button, you can use the hook_entity_presave.
hook_entity_presave(Drupal\Core\Entity\EntityInterface $entity);

Example:

function CUSTOM_MODULE_node_presave(Drupal\node\NodeInterface $node) {
  $node-&gt;setTitle('Edited Title');
  $node-&gt;set('body', 'this is the bew body');
  //CAUTION : Do not save here, because it's automatic.
}



  Note : Do not re save inside of hook_entity_presave().


Using HOOK_entity_presave()

function mymodule_entity_presave(Drupal\Core\Entity\EntityInterface $entity) {
  if ($entity-&gt;getEntityType()-&gt;id() == 'node') {
    $entity-&gt;setTitle('The new Title');
    //CAUTION : Do not save here, because it's automatic.
  }
}


Source : drupal8.ovh