As I remember it, the button is soldered to a long 1/8" wire with a bend on the end to engage the ratchet pawl at the bottom. Remove the assembly and you'll see how it works. Prying the wire link out of the pawl will allow the button and spring to come out the top. I don't have a diagram (and I'm doing this from memory
) so if you need more help, I can take a closer look on Monday.