I think it depends on the configuration. I will let Emile correct me if I mis-speak here.
Unscrew screw on top plate and remove. Of course turn of the power to the Extreme.
To get the best sound you want the new usb card in the pci 5 slot. BUT my SS drive was in the pci5 slot.
Long story short you unscrew one screw on the SSD drive that holds it in place, (small clip on bottom of the SSD drive holds it in place as well), relocate the SS Drive to pci1 slot (closest to the CPUs). Then you relocate the fiber card to pci2. slot. Each involves taking out one screw then the boards pull out easily.
Then you place the new usb card into the pci5 slot. There is a cable that comes with the card. One end connects into the card, the other end has a four pin connector that connects to a connector on the inside of the front plate. VERY OBVIOUS.
It took about 15 minutes total. No driver updates. Put the top back on, connect the usb to the new port, turn the Extreme on and hit play.
If anyone is inserting their card and has an issue feel free to give me a call, be glad to help.