You have to take the sight off the gun to replace that button. It comes off like many other sights of this type: You press it down and slide it backwards. With the sight off, you can take the slider off and replace the button. I would think that you'll find the rear half of the button and the spring captured inside the slider, still engaging the slots in the sight. You will probably have to pry on them a bit to make it possible to take the slider off.
EDIT: I take that back. The spring will not be there, it's located right behind the button so it must have fallen out together with this.
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