Whirlpool ARZ 121/H

Question

URGENT HELP NEEDED. I have a whirpool ARZ 121/H undercounter larder fridge. I can't get the temperature to settle and it fluctuates between 2 - 14oC. I need to hibernate my tortoises and need the fridge to settle between 2 - 7 oC. if it's the thermostat then i have a new one but don't know how to remove the old one and replace - HELP