With the lockbox open...make sure the latch is in the lock position.
If you don't have the tool...use a small flathead screwdriver.
Make sure all the little arrows on the lock are pointing in the same direction by lightly pressing and turning them until they are all pointing in the same direction (to the top of the box)... press the clear button down.
Select your code by turning the arrows in the opposite direction...(to the bottom of the box) should be half a turn. Then press the slide button on the front which will set the new number. Test the new code BEFORE you close the box.
Hope this helps