First, ensure that you are electrically competent to do this job.
Remember electricity kills!! Unplug the appliance from the wall socket.
Pull the thermostat dial off the thermostat. In front of the thermostat dial there is a small green button.
Prise this off with a small screwdriver, unscrew and remove the screw holding the control unit in place and pull the whole unit towards you slightly.
This will remove the unit from the wall of the fridge.
Remove the thin plastic cover from the rear which will expose the thermostat and electrical wiring. T
ake note of the wiring connections and disconnect the three wires.
Unscrew the hexagonal nut holding the thermostat in place. On the metal plate mounted on the rear wall there are five screws, three large and two a bit smaller.
The three large screws hold the plate to the rear wall and form a triangle, one on the top and two near the bottom left and right.
The two small screws hold the temperature sensor (the long thin "wire" connected to the thermostat).
Loosen the right hand large screw (this will give you a bit of room to feel the bracket that holds the sensor against the metal refrigeration plate).
Slacken the two small screws and help the help the old sensor out of the retaining bracket. remove the thermostat. Note how much of the sensor is bent back on its-self and do the same with the new one.
Re-assembley is the reverse and getting the sensor into the retaining bracket can be fiddly.
Make sure that you don't puncture either the sensor "wire" or the refriegrant pipe on the back of the refrigeration plate. If you puncture the refrigerant pipe you've not only destroyed the planet but your fridge is heading for the skip BER.
A last warning, Electricity kills. Make sure that you're competent. If not hire a tradesman. His wife or partner will welcome the extra income