Question about Update on Trigger with condition - mysql

I can't find the correct syntax for the trigger I tried to code.
I want to update a timestamp every time there is an update on the table but only for specific rows. I want to update the "UPDATE_TIME" only for the last "N_TACHE"
I can't find how to use SET with a where clause.
CREATE TRIGGER "time_on_update"
BEFORE UPDATE ON "SAVE_UPDATE"
FOR EACH ROW
BEGIN
IF
(NEW.DATE_DE_RDV <> OLD.DATE_DE_RDV)
OR
(NEW.ADRESSE <> OLD.ADRESSE)
OR
(NEW.CODE_POSTAL <> OLD.CODE_POSTAL)
OR
(NEW.VILLE <> OLD.VILLE)
THEN
SET (NEW.UPDATE_TIME = current_timestamp WHERE N_TACHE= (SELECT MAX(N_TACHE));
END IF;
If you have any suggestion I take it.
Thanks in advance
EDIT :
This one is working but it update all the rows and I just want to update "update_time" for the last value of "N_TACHE" when "RDV","ADRESSE","CODE_POSTAL" or "VILLE" is updated
DELIMITER $$
CREATE TRIGGER `time_on_update`
BEFORE UPDATE ON `SAVE_UPDATE`
FOR EACH ROW
BEGIN
IF
(SELECT MAX(N_TACHE)
FROM SAVE_UPDATE
WHERE
((NEW.DATE_DE_RDV <> OLD.DATE_DE_RDV)
OR
(NEW.ADRESSE <> OLD.ADRESSE)
OR
(NEW.CODE_POSTAL <> OLD.CODE_POSTAL)
OR
(NEW.VILLE <> OLD.VILLE)))
THEN
SET NEW.UPDATE_TIME = current_timestamp;
END IF;
END
$$
DELIMITER ;

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END IF;
END$$
DELIMITER ;