I was wondering the same thing, then decided to go ahead and take a crack at it.
There is no 'spring' per se. It's a concave disc sitting over a contact.
Okay, so here's what you do:
1) Ignore the Motorola logo itself. Using it as a point of origin, observe how the plastic it and the answer button sits in is glossy. (Mine has a blue tint.) The housing itself is a dull or flat color (mine is grey.)
2) Near the logo you should see what looks like a slot to fit a ribbon through. Beneath this is where you will pry. Do NOT put a blade into that slot tho . Basically, it's at the top of the inset, glossy plastic. With a long, flat blade beneath the top of the clear/colored plastic, pry gently and gradually. This whole piece is held firm by adhesive. Careful to keep the sticky side clear of debris (so you can re-attach.)
3) You might see one small tab emerging from each side as you pry up. This will cause minimal damage to the housing (grey in my case), but the adhesive will keep it together once your reassamble.
4) Beneath this inset plastic is a 6 or 8 sided screw. Find either a small flat blade or perhaps a 6 or 8 sided head and remove the screw. It's a long screw.
5) Carefully observe the inside as your remove the upper half of this casing. You SHOULD see a '+' shaped piece of clear plastic covering what looks like a tiny metal disc about 1/16 of an inch. It has adhesive on the side facing the circuit board. This plastic tape keeps the disc fastened loosely to the board.
6) This disc, upon closer inspection is concave, meaning it arches up in the center of the disc. This disc is what 'clicks'. It comes down onto the 'dot and ring' contact plates beneath it. I had to simply align the disc with the rings beneath it and reapply the '+' shaped tape.
7) If the disc does not 'click' when pressed on either with your finger or the button, I don't know what to do except maybe go find a used/broken HS850 and go frankenstein on it.
I hope this helps.