Removal
Gently but firmly pull down
near a corner of the trim, then work around.
The trim is snapped on near each corner (four snap points),
but there is no way to remove the trim other than pulling it
down or removing the whole headliner, which is very time
consuming. There might be a slight risk of plastic
breaking if it is very old, but it doesn't seem likely.
Install
Line it up and snap it in.
The wider part of the trim goes toward the front of the car.
Just line up the snaps, or the map light buttons, and push
it up to snap it in place.