The top bolt draws up a wedge to expand the stem in the fork's steerer tube.
(See photo here) Loosening the bolt won't do anything until you tap the bolt to drive the wedge down, thereby freeing the stem.
As a general rule, you should bring the bolt up about 3-6 millimeters, (1/8-1/4") before tapping it to free the wedge.
Do not remove the bolt. You want to keep the assembly together, otherwise the next step will to go fishing for a wedge that has fallen into the fork.