The black powder revolver front sights attachment varies from maker to maker.
Most of them braze the sight on, so this makes it difficult to remove without heating the barrel to a red heat.
This of course, ruins the bluing.
I'd suggest taking it to a gunsmith for a replacement.
Otherwise, you can try to pull the sight off with Vise-Grips, or drive the sight off with a brass drift and a light hammer.
With the sight off, you're better off making your own replacement from steel, brass, or nickle.
You can attach the new sight with a good strong SOFT solder.
Among the choices is Brownell's Hi-force 44 high strength soft solder:
www.brownells.com/aspx/ns/store/catsearch.aspx?c=1211&p=3958OR you can buy a silver "bearing" soft solder at local hobby shops and at Wal-mart.
Note that this is NOT true silver solder. It's a higher strength soft solder with about 3% silver added to increase strength and prevent tarnishing of the solder.
All these solders melt at 400 to 600 degrees, which is well under the damaging 1100 degree heat needed for real silver "solder" braze.
When soldering take care to keep the flux off the bluing since this will damage it.
For parts for these guns, the top source is Dixie Gun Works in Tennessee:
www.dixiegun.com/