If you want to modify the value that is saved you would still do this using an acf/save_post action. You would just get the field and then update it with the altered value.
You could skip the update part if you use a priority of < 10 for your acf/save_post action to happen before ACF saves the value. In this case you would modify the value held in $_POST[‘acf’][FIELD_KEY] and let ACF do the updating.
