You have to take off the water pump. Loosen the bolts, and give the water pump a punch with a hammer so it comes off. Once it fell off you can see the nipple from the inside. All you need to do is to hammer it out, from behind.
I used a socket on the nipple(if you are reusing it) from the inside out. bash the hell out of it with a hammer.
Thats how i did mine...first time i did it with a piece of wood and messed it up but that wasn't getting reused.
i ended up heating up teh water pump housing to expand it and cooling the nipple to shrink it and it just poped out.