sight adjustment
As E. BeauBeaux says, use a brass drift punch. If you don't mind a cleaning job when you finish, soaking the rear sight in some WD-40 or other good penatrating oil for about 15 or 20 minutes before you bang on it will make your job easier.
FWIW, I've seen a couple of the 226s come from the factory with the front sight adjusted way to the left of the slide. (Presumably to correct its shooting to the right.) Why the factory would prefer to shift the front sight and not the rear is a question for which I have no answer.