Ok to replace the thermostat you need to get the top off.
To do this pull oven out and look at the back there will be 2 screws 1 either end holding the top on.
You may need to remove a trim on the back at top which goes across. Then when the screws are off lift the top up at the back and slid it forwards.
Then it will lift up. Just spin it round as it wont come off completely due to the hobs on top so just spin round so you can get at the thermostat.
Then pull the knob off and there will be either a nut or 2 screws holding the thermostat on. Remove these and then you can take it out and fit the new one in.
Then follow the silver capillary down to the oven and unclip it from inside then it will come out and you can fit the new one in place.
Then refit top and thats it job done.
If you need anymore info just get back to me.