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.
Is your front sight the factory metal 2x4 ramp front sight or a fiber optic front sight?
If it's the factory metal ramp front sight, or the entire front sight frame for a fiber optic, follow Breadman03's advice.
If it's a fiber optic front sight, specifically the colored plastic tube that fell off/out, just replace it and either glue it back in place or melt the end carefully a little bit to keep in from comming back out. Be sure it's the color you like before doing this though.