Example -- Consider a FND User having following Details --

Lets Try to Update its Email Id from PRAJ_TEST@abc.com to PRAJ_TEST@def.com

-- ------------------------------------------------
-- API to UPDATE Oracle FND User
-- ------------------------------------------------
DECLARE
    lc_user_name                           VARCHAR2(100)   := 'PRAJ_TEST';
    lc_user_password                   VARCHAR2(100)   := 'Oracle123';
    ld_user_start_date                   DATE                      := TO_DATE('23-JUN-2012');
    ld_user_end_date                    VARCHAR2(100)  := NULL;
    ld_password_date                   VARCHAR2(100)  := TO_DATE('23-JUN-2012');
    ld_password_lifespan_days  NUMBER               := 90;
    ln_person_id                            NUMBER                := 32979;
    lc_email_address                     VARCHAR2(100)  := 'PRAJ_TEST@def.com';

BEGIN
   fnd_user_pkg.updateuser
   (  x_user_name                           => lc_user_name,
      x_owner                                   => NULL,
      x_unencrypted_password    => lc_user_password,
      x_start_date                             => ld_user_start_date,
      x_end_date                              => ld_user_end_date,
      x_password_date                   => ld_password_date,
      x_password_lifespan_days  => ld_password_lifespan_days,
      x_employee_id                       => ln_person_id,
      x_email_address                    => lc_email_address
   );

COMMIT;

EXCEPTION
    WHEN OTHERS THEN
                      ROLLBACK;
                      DBMS_OUTPUT.PUT_LINE(SQLERRM);
END;
/

SHOW ERR;