After you remove the sight - carefully, as Fred said - you will see a small, bright, grinder-mark, with a pin in the center. The pin is driven blind - cannot be reached from above. You will have to drill it out, or, somehow reduce (by drilling a series of small holes) the metal around the pin to where you MIGHT be able to pull it free.

Whatever drilling you do, use a drill-press and drill vise, and proceed very slowly. You ONLY need to remove that part of the pin which is in the fixed base. As soon as you can wiggle the movable base, stop drilling!

Since I do not shoot my collector Krags, I've never worried about it.

Or, you could buy an unpinned sight from a parts supplier - 1901 sights are not uncommon; you should be able to obtain a good finish match. The parts guy might take your sight in trade.