The Platform Keyboard Tray features two different height settings allowing different clearance. The standard tray setting offers 4.25" between the tray and desktop, while the lower setting gives you 5.75" between the two.
Looking at the tray wings you'll see two silver L-shaped brackets that attach the wings to the desktop. To lower the tray please remove these screws and attach the bottom two screw holes of the silver bracket to the top 2 holes on the wings.
For further clarification, you'll still have all 16 screws attaching the brackets to the underside of the desktop - the screws being removed are perpendicular to the force of your playing and won't compromise the overall safety of the keyboard tray.