The sides slot in to the walls. If you push up on the headliner pieces, you'll see that there are "tabs" about 4" (?) long that slot into holes in the metal van wall.
First, undo the clips in the top piece of the headliner. You'll need a
plastic trim removal tool to unsnap the plastic clips that hold the headliner against the metal ribs in the ceiling.
Once you've done that, the headliner goes a bit more floppy. With the extra play this gives you, you can push the headliner up at one side to make the tabs in the side come free. Now the other side just lifts out.
I think some of the threads that address this say to start from the back of the van, but I think it's easier to start at the front (the piece just behind the cab).