Ambient Switch on Fridge Loose & Not Responding?

The type of inquiry : Product Operation
Product/Model No. : Top/Bottom Mount/GC-259Y

The Switch on my thermostat for turning this on when the ambient temp is lower than 18C HAS BECOME LOOSE AND WILL NOT RESPOND, IT WAS STUCK AT ONE POINT ENTIRELY - AND I TAPPED THIS - TO FREE IT, BUT THIS HAS SUBSEQUENTLY LOOSENED IT 'ENTIRELY...,' so it does not respond at all; how can I fix this please?

I'd like to be able to take off the plastic cover for this please - "for a start" but not sure how to do this, without damaging it? Getting the light open is easy enough but this is the main plastic panel the switch functions are behind, but the switch has rarely worked correctly, so if you can do anything to assist please? I think the switch requires replacing.
Looking for easy and simple to instigate instructions please, as money is tight...?

Your Thanks