Ed, I have replaced roof lights on a van previously, and they are pretty straightforward.
Usually come as 2 parts, inner and outer.
Unscrew the old inner from the outer, remove the inner, then use a wide flat blade to break the seal on the outer at the roof, remove old outer.
Thoroughly clean the area of old sealer, apply a good bead of silkaflex to the new outer, slide into the aperture, go inside the van, offer up the new inner and screw the two parts together, job done