Update a node / Entity programmatically in Drupal 8

Update a node:

Example 1 : Update title

$node = \Drupal\node\Entity\Node::load($nid);

$node->setTitle('The new Title')

$node->save();

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

$node = \Drupal\node\Entity\Node::load($nid);

$node->set("body", 'New body text');

$node->set("field_name", 'New value');

$node->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->setTitle('Edited Title');

  $node->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->getEntityType()->id() == 'node') {

    $entity->setTitle('The new Title');

    //CAUTION : Do not save here, because it's automatic.

  }

}