CASE or IF or WHERE inside UPDATE

UPDATE `table_name`
SET `var`=1
WHERE `condition`='value' AND `var`=0

//or

UPDATE `table_name`
SET `var`= CASE
WHEN `var`=0 THEN 1
ELSE `var`
END
WHERE `condition`='value'

//or

UPDATE `table_name`
SET `var`= IF(`var`=0,1,`var`)
WHERE `condition`='value'