Unplug the refrigerator from its power source. Move it away from the wall so you have access to the panel on the bottom of the back of the fridge.
Remove all screws holding the panel in place and remove the panel to locate a large metal box covering the condenser assembly.
Take out the screws on the box, then remove the box to expose the condenser fan and motor assembly.
Unplug the wiring running to the ports in the condenser assembly.
Take out the screws on the condenser assembly and remove the assembly.
Fit a new condenser assembly into the old one's place, after ensuring it's compatible with your particular brand of refrigerator.
Replace the screws and plug the wiring into the ports on the assembly.
Replace the metal box and screws. Replace the back cover and screws.