change the value of an associative array attached to an object by any array key

gistfile1.php

<?php
define('LANGUAGE_NONE', 'und');
function _update_array_value($object, $field, $key, $value, $lang = LANGUAGE_NONE) {
  foreach ($object->{$field}[$lang] as &$sub) {
    if (isset($sub[$key]) && $sub[$key] == $value[$key]) {
      $sub = $value;
    }
  }
}

$node = new stdClass;
$node->field_entityreference[LANGUAGE_NONE] = array(
  array('target_id'=>1),
  array('target_id'=>2),
  array('target_id'=>3),
  array('target_id'=>4),
  array('target_id'=>250),
  array('target_id'=>262),
);

$key = 'target_id';
$value = array(
  'target_id' => 250,
  'date' => '2014-03-21 18:13:20',
);
_update_array_value($node, 'field_entityreference', $key, $value);
print '<pre>';
print_r($node);