Most engine covers on GM vans come off in the same general way. Usually you have to first remove the lower instrument panel extension pieces, held by some screws, washers, etc.
Then you unclamp the engine cover (usually two clamps on either side of the cover, up high) , and then dig around the perimeter of the cover, maybe under the carpeting, for lock screws or bolts that hold the cover to the floor. Then, with all that disconnected, it should pull right out.
When you put it back you have to be sure that your insulating strip/gasket, is in good shape so that you don't get engine fumes into the interior of the van.