I have two entries grouped but only one is changed in the update

I've mounted an update by grouping the presenca field that should change from two arrays to one repeat loop, follow query :

UPDATE acessos AS a
  JOIN
    ( SELECT usr_id, presenca
      FROM acessos
      GROUP BY presenca
    ) AS c
    ON  a.usr_id = c.usr_id

SET 
    a.presenca = 1
    WHERE a.usr_id = 2

It only works with usr_id = 10 , what can it be?