Perhaps there is a post about this already. Since I was changing straps on my Oris, might as well take some photos of it showing the puck in action.
Flat head screw driver from one of the glass vial, note the flat notch is aligned with the flat head of screw driver.
Place the screw driver in the puck, line up the flat side with the set screw on the side of the puck.
Slide it down until the bottom is flush then screw in the set screw to secure the flat head.
Align this flat head with the screw inside of the lug. If you wish you may want to tap up the surrounding on the lug. The tolerance is pretty tight, not much slop there.
Align the screw in lug hole with the flat head on puck. Gently rotate the puck slightly to make sure the flat head is properly seated in the screw. Once seated, press down with some pressure then use the free hand with the watchmaker's screw driver to unscrew the screw on the opposite side lug. Sorry no pic here, as I'm out of hands at this point.
Once the screw is removed, a trick is to use the spare tube to push out the tube in the watch/band.
Out it comes.
Wear it in good health. :)