Really? it's quite simple, figured it out all on my own YEARS ago when I played at the arcade. I had to help the arcade replace a few bad buttons. The only thing in the cabinets that I wont touch is the Monitor stuff. VERY dangerous, the rest is simple.
1) Flip control panel so you see the bottom
2) Remove white cherry switch (this is actually what triggers the action, label it with a marker) by pushing out one of the two mount pieces. It has two wires that run to the game board.
3) Unscrew the black plastic nut from the base of the bottom side of the control panel. These shouldn't be too tight, I usually have to hand tighten them in most cases now and then.
4) Button should now slide out of control panel.
To add a new button to the board is harder without a proper harness, I can tell you where to buy those pre-made instead of having you do that part. Will cost about $15 and two new Yellow Buttons will be about $3-5 each. I usually buy some parts from Happ Controls since they are local and I used to work there one summer, but another good place for cheaper and custom made/obscure parts is Bob Roberts He has a more manual way to order stuff. From him you'd need these parts:
Are you missing the joysticks too? If so get these parts
1 Jamma *Plus**Kicks**Extra* harness complete $15
2 Happ Microswitch Pushbuttons Y $1.25 each (sale)
2 Happ Ultimate Joys 8-way $8.00 each (sale)
It should be about $8 in shipping, you mail him a check and he turns around and mails you the stuff. Little slow, but it's worth it.
2 Happ Ultimate Joys 8-way Red $8.00 each (sale)
1 Set Replacement Cherry Microswitches .187 tabs 10 for $9