I personally just took out the top screw, then very carefully pry'd his mouth/upper lip open just a hair with a small flathead screwdriver. Then slide in the fangs and they should hold.

There's a bolt in the back also that you need to get off.
After that, it's in pretty tight, so you'll need to wiggle it off.

Use the fattrain approach, then if you can't get enough wiggle room, do what bowz said.

That's also the named approach I use on my girlfriend

Yes-if you can, you should def try to just slide them in.
I bought a modded snakehead, so the old head had to go.

If the snake head itself is anything like on the Pro, there's a screw underneath the top mouth roof area (towards the front of the mouth top, underneath) - it goes through the metal bracket (phillips head). Feel underneath with your finger - if you feel a small round circle with a screw head in it, it's the same as on the Pro. Don't pry the head up if you can avoid it; you may break the attachment point for the screw inside the head.

edit : I have used a small ratchet with a very short (3/4 inch) philips head bit to unscrew the screw.

Metallica : teaching pinball enthusiasts dentistry since May 2013.

The (premium) mechanism is a PITA to remove. I decided to shim the mechanisms with a few washers to lower the jaw and hopefully reduce the number of shot rejects to the mouth. I don't think the mech can be removed with the top head on, not unless you also remove the edge protector in front of the jaw. There just doesn't seem to be enough room. The top head is held on by 3 screws and 2 are impossible to get to without some level of further disassembly. I don't know how you get at the one in the roof of the mouth without removing the lower jaw switch assembly.

It's a neat mechanism, but not fun to work on.

Edit: Actually the snake head does come out without removing the top. It just needs to be angled right. Pull the back end down first, angle the jaw up and it should barely come out.
Rob