update salary set sex=IF(sex='f','m','f');
or
UPDATE salary SET SEX = CASE WHEN SEX = 'f' THEN 'm' WHEN SEX = 'm' THEN 'f' END ;
or
update salary set sex = char(ascii('m') + ascii('f') - ascii(sex));
update salary set sex=IF(sex='f','m','f');
or
or
update salary set sex = char(ascii('m') + ascii('f') - ascii(sex));