Disconnect the cables (2 to the computer, 2-3 down to the cabinet), and grounding wire.
Leave out the rear panel and also take out the backglass. This will make it easier to grip the head.
While there, make sure the metal box for the computer is secure. There are holes on the top corners of the box where you can use zip ties, stiff wire, rope, etc to secure the box to the mounting bracket.
Slide out the playfield glass and put it far away from the game (don't lean it against the game in case something goes wrong). There are a pair of bolts on each side of the head. The bolts are really two halves of a bolt that screw together.
Both sides have a head that has a hole for an allen key. They are slightly different sizes. If it's been years since they were last unscrewed, it may take a lot of muscle. I use a 7 pc set of allen wrenches with t-handles to take them apart; the sizes in the set are 1/4", 7/32", 3/16", 5/32", 1/8" 7/64", 3/32". I don't recall which two were needed, but they're among those sizes. T-handle wrenches are better to use to prevent damage to parts on the playfield in case something slips.
Once the bolts are removed, the head will pretty much stay where it sits.
The best method I've found to dismount the head is to move it in two stages since it's hard to reposition your grip to lift up then lower to the floor.
First, have a table or lift cart (as high as it will go) to place the head on. It's usually easiest to have the lift cart behind the game.
Then lift the head off the game, tip it backwards so it lays on its back. Just be careful about placement since the back door is still out.
Then lower the lift cart. Put down cardboard or packing blankets on the floor and tip the head over on its side. Put the doors and backglass in. Tip/lift the head back onto the cart, wrap it up with stretch wrap, and cart it away.
Put the glass back into the body cabinet. Note: don't use stretch wrap directly on the glass--it will leave marks that are hard to remove. Put down a blanket or a sheet of cardboard on the glass first before wrapping the game up in plastic. Other than that, treat the body cabinet like any other game.