Take them off the car (2 screws in the engine bay, one behind the bumper on the side for each). Take out all the bulbs and the small gold screws on the back. Preheat an oven to 200 degrees. Turn the oven off and put the headlight(s) in for 15 minutes. Get a screwdriver or similar flat object and pry and pull around the edge of the clear top lens. The glue will be soft enough for you to separate it. If not, put them back in the oven for another 15. The textured clear lens inside comes off with 2 silver screws, then just pry it off (will make a loud pop noise; don't be scared ).
To put them back together, just press the clear lens onto the rest of the housing and put them in the oven again. Then, squeeze them together a bit more and put the gold screws back in.