// The EntityManager, em, already exists for this example.
...
// We just happen to know that '5' is one of the
// player identifiers. You shouldn't normally hard-code this
// into any application.
Player p = em.find(Player.class, 5L);
em.clear();
// p is now detached for the convenience of this example

Team t = new Team("Ventura Surfers", "National");
p.setTeam(t);

em.getTransaction().begin();
Player managedPlayer = em.merge(p);
em.getTransaction().commit();