Originally Posted by FBinNY

There's a wedge attached to the bolt which you push down so the stem releases. See image

It only needs to drop an 1/8" or so, so if the bolt is attached and you can freely lift and drop the bolt that much, you're done. Leave it attached so you don't have to go fishing for the wedge later.

Once the wedge is free, the stem adapter can still be snug in the steerer. Back when quill stems were SOP, you'd typically twist the bars left and right with the wheel trapped between your knees and work the stem up. So, clamp your bars back onto the adapter and do the same thing.