Keep the temperature low and replace that broken fan!
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 near the bottom of the iMac.
Pull the rear panel toward yourself and remove it from the iMac.
Remove the two 9.7 mm shouldered T8 Torx screws from the left fan.
Disconnect both fans by pulling their connectors straight up off the logic board.
Lift the left fan out of the chassis, minding any cables that may get caught.