how can i fix my thermol sesor swich on my corsa 1.2 M456 KAV?

my car is using to much petrol,the yellow/orange englin light comes on and off now and again,the car over rev's and cuts out on me and wont start staight away,some one told me that i need a new thermolistic sesor switch as the old one hasnt been put in properly does anyone kno how to fix this?