Go through the door and into the office ahead. You'll find Wheatley yelling at some of his nightmarish creations: frankenturrets. These seem to be cubes fused with a couple of turrets, and they don't seem to be following orders very well.
There is a broken vent in a corner of the office. Make a portal on the wall that you can see through the broken vent, then make another on a nearby wall. Go through. Go to the end of the walkway and get into the yellow elevator.
Pick up a Frankenturret and put it on the big red button. After a bit of dialogue between GLaDOS and Wheatley, go through the exit door. Follow the path to the elevator. On the way, you'll learn that Wheatley now has an urge to conduct tests, which apparently is hardwired into GLaDOS's old body, which he now occupies.