Stage 1, Head removal:

You just can't pull off HK's head as is, no matter how hard you try. And here you can see why, the neck joint is tapered at the top, but flat at the bottom, like a lamp shade. So what you'll have to do is pull the head back far enough in order to cut a small slit on the bottom of the head with an xacto knife. (Do it on the back side, and don't cut yourself in the process.) This should widen the hole just enough to pull the head off.

This is the hardest part, the rest is easy.