Easily replace your AirPort Extreme card to regain wireless connectivity.
Lay the iMac display-side down on a flat surface.
Loosen the three Phillips screws securing the rear panel to the iMac.
Lift the rear panel slightly from the bottom edge of the iMac.
Pull the rear panel toward yourself and remove it from the iMac.
Lift the AirPort card slightly by its plastic tab and pull it straight away from its socket.
Insert the flat end of a spudger between the body of the AirPort card and its antenna connector.
Push the spudger away from the AirPort card to disconnect the AirPort antenna.