Not the easyist thing to replace im afraid but to access it you need to pull the oven out and remove the back panel.
Then you need to remove all the pan stands on the hob and all the burners.
Then remove the screws either side of all the burners.
Then remove the screws holding the hob top on at the back.
On some models theres a screws on the side at the top. If so remove this as well.
Now you should be able to remove the complete hob top.
From there you can then access the thermostats and switches and swap them over from there.
Hope this helps but to access the thermostat the back and the complete hob top will need to be removed im afraid.