Got a good solid wood work bench that you don't mind nicking the top edge up on it?
Got a dowel rod that will fit into the barrel hole in the front of the slide?
Got a hammer?
Put the dowel rod down in the slide, put the back of the hammer over top/against the corner of the bench and wack the dowel rod with the hammer.
Be sure not to drop the slide.
Once it starts to move out of the slot in the slide just a little bit, you won't have to hit it so hard to keep it moving and finally get it out of there.
When you install the new front sight it may be pretty darn tight in the slit. Don't sand/stone/file the sides of the front sight, instead, remove metal from the bottom of the front sight. It won't take much. But it's way easier to remove metal from the bottom of the sight (and not scratch the sides of the sight itself) than to remove metal from the sides of the sight that fits into the slot in the slide (BTDT).
Good luck with it.