Get yourself one of these:
http://static.speedwaymotors.com/RS/...91089434_L.jpg
take off all the bolts except for a few at the top, but loosen those so you don't accidentally pop the window. Cut between the plastic frame and the van body in between the bolts all the way around. It'll give, then just kind of pry it off. Clean up all the butyl on the van body and off the frame and then replace with new butyl. One trick I learned is to leave the butyl round around the bolts and smoosh it flat a bit between the bolts. It will give you a better fit when you reinstall. The bolts will smash the butyl right around them.
Put it in place and then tighten the bolts to snuggish. Do it incrimentally around and around so as to get and even seal.
I just replaced three windows I messed up and put in rear popouts. It's not so bad. 3M window weld is what I got. good stuff.