The pin is a roll pin, actually a "spring tension pin", that is pushed out with a small punch. Usually you can re-use the pin.
If you want to do it yourself, it's not hard and here's how.
Buy a .039 or 1/16" pin punch from Brownell's. In a pinch you can cut off a drill bit and use the shank as a punch as long as it properly fits the hole in the frame and sight.
Put the gun on a clean table with the left side up.
Use the punch and a small hammer to tap the pin out from LEFT to RIGHT. You don't have to completely remove the pin, just drive it out until the sight leaf released and moves upward out of the frame cut.
Use a screwdriver to unscrew the elevation screw until it comes off the frame.
IMMEDIATELY wrap some tape around the center of the sight leaf. The cross pin that retains the elevation screw can fall out and release the the elevation screw, loosing the two super tiny balls and the tiny spring.
Also don't lose the two small springs under the sight leaf.
Make sure the springs are in the holes on the under side of the sight.
Carefully start the elevation screw into the threaded hole in the frame. Take care not to cross thread it.
Screw the elevation screw down most of the way.
Assuming you didn't drive the pin all the way out.....
Press the front of the leaf down into the frame cut and insert the pin punch from the LEFT side to align the holes in the sight and the frame.
Use a small hammer to gently tap the pin back in place, making sure it properly enters the hole through the sight leaf.
When the pin is close to flush with the frame, use the pin punch to tap it in until it's centered in the frame holes.
If you want a perfect job buy a roll pin punch from Brownell's this prevents any damage to the roll pin ends.
If you don't want to do it yourself any competent gunsmith can do this. I don't know of any in your area.