Speedometer gear assembly

[wrndln]

I have a decent speedometer gear assembly shown in the picture below, however the gear in it shows a lot of wear. I tried to search the archives, but I probably didn't use the correct search word. Anyway, gears are available at parts suppliers, but I don't know how to remove the shaft to replace the worn gear. If someone that has replaced the gear could explain how the gear is removed and replaced, I would appreciate it.
Thanks
Rusty Nelson