How to implement update () method in DAO using EntityManager (JPA)?

What is the standard way to implement simple update?

Example: we have User with phone number NNNNNN and now we want to set it to YYYYYY.

@PersistenceContext
private EntityManager em;

public void update (User transientUser) {
    what should be here?
}

User entity is as simple as possible:

@Entity
@Table (name = "USER")
public class User {

    @Id
    @GeneratedValue
    private Integer id;

    @Column (nullable = false, unique = true)
    private String login;
    private String phone;

    public User () { }

    ... //some setters and getters
}