API - fnd_user_pkg.updateuser 
Example -- Consider a FND User having following Details --
 
 
Lets Try to Update its Email Id from 
[email protected] to 
[email protected]
 
-- ------------------------------------------------
  -- 
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)  := '
[email protected]';
  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;