You should be able to do this in a single update

UPDATE Geo_postaggregated_pop_count_Ward
SET Sum_male_count =
        (SELECT SUM(G.Person_count)
           FROM Geo_postaggregated_pop_count_Ward G
          WHERE Sex = 1),
    Sum_female_count =
              (SELECT SUM(G.Person_count)
                 FROM Geo_postaggregated_pop_count_Ward G
                WHERE Sex = 2),
    Sum_of_person_count =
              (SELECT SUM(G.Person_count)
                 FROM Geo_postaggregated_pop_count_Ward G
                WHERE Sex = 1 OR
                      Sex = 2);
[download]

This assumes you can afford the large lock this would generate. Otherwise use a Transact SQL cursor.

Hope it helps
UnderMine