Updating objects is very easy, you just call the Doctrine_Record::save() method. The other way (perhaps even easier) is to call Doctrine_Connection::flush() which saves all objects. It should be noted though that flushing is a much heavier operation than just calling save method.
<code type="php"> $table = $conn->getTable("User"); $user = $table->find(2); if($user !== false) { $user->name = "Jack Daniels"; $user->save(); } </code>