Having replaced or assisted in about a
dozen canvas replacements over the years I find yanking the top off a time
saver. With three people pulling the top only takes 15 minutes. Then the canvas 
can be centered and put on the top while comfortable. If
you don't have the man power to yank the top it's still
manageable to do on the van as others have done. 
Couple tips:
Pull a couple screws and get a bunch of spares from the hardware store. 
Mark the locations where any screw holes are over sized 
so when the canvas and metal brackets are off you can
put a dab of wood glue in the hole. That way when installing 
the new canvas the threads will grab. 
Make sure the canvas isn't inside out. Did that once on a
buddies van who is very very particular. He was upset
and thought we ruined his tent but it was fine. 
Use painters tape to hold the tent up while putting the top
back on the van. 
Make sure you seal any bolts you remove that go through the top so it don't leak. 
A new canvas is one of the more affordable things you can do that will really spruce up the vans looks and usefulness. 
Last,don't try and patch an old rotten canvas thinking it will
hold your kids up there. I witnessed the aftermath of a five year old that leaned up against the canvas in the middle of 
the night and fell out. I ended up driving the child and father 
to the ER in my van as we were pretty much packed to
leave early and it would have taken them a long time to
breakdown. Five staples and a concussion were the end
result. Worst part is he had the canvas under his seat but 
never got around to install it. 

Bill

On Jul 23, 2012, at 3:48 PM, Tom Carchrae <tom@CARCHRAE.NET> wrote:

> 
> Taking the top off seemed like it would have been much more work.
> 
> Only downside on our job is a few screws were broken, but fortunately not
> badly enough to make it loose.  Not clear if it was us or the previous 30
> years that did it.
> 
> Tom