You are lucky, yours unscrewed from the floor plate. Mine was frozen to the floor. That black button actually disengages that ratchet that is suppose to keep the post tight in the floor plate. mine was so tight I could not depress it at all. The top is just a tight friction fit to the post. If you turn the top in the clockwise direction while pulling up it will come off the post then you step on the black button while turning the post counter clockwise and it will unscrew from the floor. You may find that the first couple times you do this that removing the top will tighten the post so much you can not unscrew it from the floor. You can then put the top back on and turn it counter clockwise to help loosen the floor screw. After a few times things will loosen enough that you can get the top off without too much trouble.
There is nothing in the manual about this and I had to remove the floor plate with the pole stuck in it to see how it worked. There is a round toothed ring on the bottom of the post and the button is attached to a spring loaded wedge that engages the toothed ring.
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