Can I change the sights on my Grand Power MK23 at home?
Yes, with a sight pusher or a punch. Drift the old sight out of the dovetail and push the new one in. If you do not have the tool, a gunsmith can do it for you.
Do I get both the front and the rear sight?
Yes, unless the title says "front sight" or "rear sight". Then you get only that part.