Excellent question--I had to ask the first time, too.
This piece of inside skin just "oil cans" out. You need to remove all the pop rivets--there aren't many. Be careful if there are pop rivets at the ends that are in the outer three inches, eg, towards the edges. All the wires run in that area. Then take out all the inside vent covers/lights.
Once you get the pop rivets out, you'll have to figure out a way to start popping the skin downwards in the center. If you are doing it alone, get a long pole/stick and slip it through one of the vent openings as far as possible. Then you can leve down a longer length of the skin. If you don't do this, you'll get a short section to deflect/bow downwards, but you'll create a crease in the skin approximately where the downward deflected skin meets the upward deflected skin. The long pole will allow you to carefully deflect a long run of the skin downward, making a more shallow change of deflection where the change of camber occurs and avoiding the crease.
Here's a couple of threads on the subject:
It's a little scary, so be careful, but it does work.