How to fix a thermostat ?

I have a Zanussi built in fridge freezer Model ZXC 78/31 and want to know exactly to undo the existing thermostat and replace.
I have taken out the single screw but the item does not want to shift.
Additionally how and where to fix the other end of the pipe. - which presumably is part of the new item - which I have yet to acquire. I need to know the ins and outs of fixing before I lay out the cash !