How to copy column data from one table to another table

You can also do with a for:

BEGIN
  FOR rec IN (SELECT bk.email
                    ,uc.id_consumidor
                FROM uc
                    ,backup bk
               WHERE uc.idconsumidor = bk.idconsumidor)
  LOOP
    UPDATE uc
       SET uc.email = rec.email
     WHERE uc.idconsumidor = rec.id_consumidor;
  END LOOP;
END;

In the for query I have already done the join between the tables, so I assure you that it will only bring records that really need to be changed. Inside the loop I do the update.

UPDATE uc u
   SET (email) = (SELECT b.email
                  FROM backup b
                  WHERE u.idconsumidor = b.idconsumidor)
WHERE EXISTS (
    SELECT 1
      FROM backup b2
     WHERE u.idconsumidor = b2.idconsumidor)

WHERE out (2nd) exists to prevent you from updating records in the u table that do not have corresponding in the backup table. Without this where, unmatched records would have the email column set to NULL.

The script to update the email from the backup table can be done as follows:

/* AMBIENTE DE TESTE: Tabelas temporárias para testar o script */
create global temporary table tmp_uc
(   
    idconsumidor int,
    email varchar2(100)
)
on commit preserve rows;

create global temporary table tmp_backup
(   
    idconsumidor int,
    email varchar2(100)
)
on commit preserve rows;

--Dados para teste
insert into tmp_uc (idconsumidor, email) values (1, 'AAAAA@uc');
insert into tmp_uc (idconsumidor, email) values (2, 'BBBBB@uc');
insert into tmp_uc (idconsumidor, email) values (3, 'CCCCC@uc');
insert into tmp_uc (idconsumidor, email) values (4, 'DDDDD@uc');
insert into tmp_uc (idconsumidor, email) values (5, 'EEEEE@uc');
insert into tmp_uc (idconsumidor, email) values (6, 'FFFFF@uc');
insert into tmp_uc (idconsumidor, email) values (7, 'GGGGG@uc');
insert into tmp_uc (idconsumidor, email) values (8, 'HHHHH@uc');
insert into tmp_uc (idconsumidor, email) values (9, 'IIIII@uc');

insert into tmp_backup (idconsumidor, email) values (1, 'AAAAA@bkp');
insert into tmp_backup (idconsumidor, email) values (2, 'BBBBB@bkp');
insert into tmp_backup (idconsumidor, email) values (3, 'CCCCC@bkp');
insert into tmp_backup (idconsumidor, email) values (4, 'DDDDD@bkp');
insert into tmp_backup (idconsumidor, email) values (5, 'EEEEE@bkp');
insert into tmp_backup (idconsumidor, email) values (6, 'FFFFF@bkp');
insert into tmp_backup (idconsumidor, email) values (7, 'GGGGG@bkp');

/* Atualização dos e-mails na tabela temporária tmp_uc para fins de teste */
update 
(   select
            u.email as email_U,
            bk.email as email_BK
    from tmp_uc u
    inner join tmp_backup bk on u.idconsumidor = bk.idconsumidor
    --where (...)
) t
set t.email_U = t.email_BK

select * from tmp_uc;

commit;

/*
idconsumidor   email
    1          AAAAA@bkp
    2          BBBBB@bkp
    3          CCCCC@bkp
    4          DDDDD@bkp
    5          EEEEE@bkp
    6          FFFFF@bkp
    7          GGGGG@bkp
    8          HHHHH@uc
    9          IIIII@uc
*/

If the script worked the way you want, you can use the following script for your physical tables:

-- Atualização dos dados na tabela física
    update 
    (   select
            u.email as email_U,
            bk.email as email_BK
        from UC u
        inner join backup bk on u.idconsumidor = bk.idconsumidor
        --where (...)
    ) t
    set t.email_U = t.email_BK

    commit;

I think the COMMIT was missing at the end:

update uc 
set uc.email = backup.email
from backup
where uc.idconsumidor =  backup.idconsumidor;

commit;