it' not fun, you have to remove the logic board since the modem is on the underside of it. This would include removing the bottom of the iMac, the metal shield, disconnecting the hard/optical drives, removing about 10 screws, removing the logic board, then flipping it over, removing another screw or two, pulling off the modem, then struggling for aminute to get the new one to line up, then reverse the whole process and hope it wasn't a software issue...