You don't want to do this until you're ready to actually make the change; it's possible that when you undo the screws that the plastic will give up the ghost.... and you want your engine to have something to sit upon until you're ready!
That said,
There are eight screws that hold on the cover plate (and that's all they do.) Remove these as carefully as you can. Remove the cover plate. Observe grease (or not) on curved bit that fits under the gear. More about grease later.
(for the rest of this discussion, we're assuming the locomotive is upside down in some kind of cradle or support, so you can actually see what's inside... so when I say "Down" it's actually "Up" with respect to the locomotive!)
If you look down into the truck, with the cover removed, you'll see that there are two screws, one at the front, and one at the back that you can now see... slightly offset from the centerline. Remove these. The truck will now come away from the top plate.
I don't have metal trucks for mine yet, so I'm not sure if the new ones come with a new cover plate. If not, you're ready to go. If they do, you can remove the cover (bolster) plate like this:
There is a large phillips screw in the center of the bolster, in the middle of the cover plate you've just revealed. Removing this allows you to remove the cover plate. There are two little brass contacts on the top plate, held on with two tiny screws. Remove the screws, and the contacts will slip out through the slots if you're careful. (Make note of which one was where.... it's important.)
You can see most of what I've just described illustrated on George Schreyer's site:
http://www.girr.org/girr/tips/tips1/shay_tips.html#newtrucks
One last point: The 36 ton shays were originally built with the idea that the trucks were independantly powered (if you put one on the track, it'll run by itself) as well as feeding power up into the engine. The newer ones are designed for DCC operation, and have one set of wires to feed power up to the engine, and another to receive it back down again. Having not installed the metal ones myself, I'm not sure whether they need to be modified for the older Shays or not ... hopefully someone can chime in and say for certain.
Matthew (OV)