First, clear the pistol. I leave the cylinder open just for extra insurance against bad habits.
On the front of the sight base, there should be a pin that you can press in to release the sight. You should be able to push in the pin and insert the blade, unless the blade is broken.
If you're is an Alaskan, there is a metal pin pressed in from the side that passes through the very bottom of the blade. Check out Levergun4570sbl on YouTube, as his videos are where I learned about how to replace the sight.
I don't know what the best method to repair a broken sight.