The video linked in the post above shows it perfectly. I have used Astros and the hammer for a while now and apart from initially suffering the unscrewing issue of the centre pin(no doubt locking ring related as it's not reverse threaded) I had no problems with the button. What I mean is, when it came apart or I took it apart it always went together easily.
i should add though, in the case of the hammer which shares the same button design, the spring was really way too light. I swapped mine out for something stronger. I know a few others did the same. There is always the possibility whoever you bought yours off did the same. It really shouldn't be at all difficult to put back together, it's a simple switch arrangement.