It's a combination of hooks, tabs and pins, and you have to undo them in a certain order to prevent breaking/damaging the hooks and tabs. My experience is with the MS6 sills, but they should be the same as the regular 6.
If I remember right, start with the forward end of the plastic door sill assembly (towards front of car). Pull away the forward inside lip (the inside edge that's against the carpet) . This should disengage the small hook that curls underneath the metal flange of the body. This will allow you to start pulling up the forward end of the sill. As you pull up you will feel resistance. This is caused by the metal spring pins/clips that fit into the holes in the door sill. Once you get to this point, pull hard to disenage the pins/clips. There should be two, and the second will pop out as you pull the plastic sill up higher.
You should find that the plastic sill will still not come out. It's held at the back by a fairly long plastic pin and hook (both molded into the plastic sill). Get your finger under the inside edge (similar to the front) and pull the corner away towards the inside of the car. This will disengage the hook and allow you to pull the back up, clearing the plastic pin out of it's hole. The sill should now be complete free of the car.
Installation is in reverse order. Engage the rear pin and hook, then line up the metal pins/clips. pound them down firmly into the holes, seat the front of the sill and make sure the hook engages on the inside edge.
