Use a spudger to loosen the right speaker cable's connector from its socket on the logic board.
It is useful to push downward on both short sides of the connector to "walk" it out of its socket. Be careful with the corners of the connectors, they can be easily broken off.
Pull the connector downwards to remove it from its socket.
Pull the right speaker straight up about an inch, toward the top of the iMac.
Lift the right speaker straight up and remove it from the iMac. This may take some force, both hands and rocking the speaker right and left to get it out.
Tilt the top of the logic board away from the rear enclosure.
Lift the logic board straight up and out of the iMac.
Be careful not to snag the board on any of the rear case's screw posts.
The I/O boards at the bottom of the logic board will be the greatest challenge. It is recommended to pull gently to avoid any damage.
During reassembly of the logic board, pay attention to the position of the I/O connectors. When the board is back in the case, insert a USB or Thunderbolt cable into one of the connectors to align it perfectly.
I successfully upgraded the Wi-Fi Bluetooth card in my late 2012 iMac to 802.11ac following this guide! The new card is now the same as what´s in the new 2013 iMacs, I used the 2013 iMac teardown to find the right part:)
Do I need to remove/clean the paste that locks the card before installing a new one? If I use a used one, what kind of paste do I have to use to replace the old one?
I just bought a new BT card (a >2013 version!) and it works!!!! Just some advise before you start: follow the instructions above including step 28. At this point unscrew the big sized mainboard screws. This allows some movement so you can pull back the motherboard if needed and will avoid straining it to much. you can nog pull out the Bluetooth card without dismantling the whole computer! Putting the new card back also works fine this way. This Saves you massive time!