Any one know how to change the thermostat on a Hotpoint ff72p fridge/freezer?

The thermostat is basically the knob that you use to set the required temperature. Take the knob off, take the cover off, and you will see a switch with 2 wires on it plus an earth connection. Remove the wires, remove the fixing nut and follow the copper tube to the end, removing it as you go. You should finish up with a switch in your hand that has a copper tube attached to it. Take it to your local spares place, get a replacement, and fitting is a reverse of removal. Should take no more than half an hour.