useualy the same way you got in. When i played through that part i got kicked out, and i had to find his spare key hidded under a rock (look in his back yard away from the roads) or you could lockpick his door (lvl 2 i think), then use the teleporter pyramids to enter the back room (the lamps are switches, save before trying this or you could get stuck) remember to drop the pyramids before useing them.