Bring a good drill with good bits, a box cutter with extra blades and a hacksaw or sawxall. Strong wire for cutting might be a idea too.
It's going to depend how the high roof was affixed, if its a reverse flange or just the screw and glue method.
I would imagine reverse flange would be quite easy, remove the bolts and use some wires the cut the epoxy? I've never removed with a reverse flange though.
For screw and glue method you basically un drill are the screws that secure the hightop to the frame of the van. It would be a good idea to keep a few of the screws so you can use them as a reference when you but new ones to reinstall it.
The high top will usually have this metal trim around where the screws go into, mine was in 3 pieces. Make sure you label what piece is where, we didn't and probably spend a good hour trying to figure out what piece went where by trying to line the holes up (we installed it about a month or two after we removed it).
Once the screws and trim piece is removed you can use the box cutter or wire to separate it from the original roof.
Would highly recommend at least 2 people, 3 would be better.
Here some photos on my build thread, I didn't document the removal too much but there are some pictures of the re-install process.