Drupal 8: Attach terms to entity programmatically

ExampleAttachTermsProgrammatically.php

<?php
use Drupal\node\Entity\Node;

/**
 * Before attaching a term(s) to a term reference field,
 * Must know:
 *   - field_example_name: the full name of the term reference field
 *   - tid: the term ID(s) to attach
 *
 * Keep in mind that this example uses Node::load() 
 * but you can use any Entity::load()
 * e.g. User::load(), Term::load(), etc.
 */

// Example 1: attaching a single term
$node = \Drupal\node\Entity\Node::load($nid);

// Attach only one term
$tid = 1; // The ID of the term to attach.
$node->set('field_example_name', $tid);
$node->save();
// End of Example 1 />

// Example 2: attaching multiple terms
$node2 = \Drupal\node\Entity\Node::load($nid2);
// To attach multiple terms, the term IDs must be in an array.
$multiple_tids = array(1, 2, 3); // Each is Term ID of an existing term.

$node2->set('field_example_name', $multiple_tids);  // Note that field_example_name must allow multiple terms.
$node2->save();
// End of Example 2 />