One takes it that this is a Cookworks microwave EM7
Exactly which switch do you wish to change? As far as I can see there is only a keyboard on the front, which would have to be changed entire, IF you could get a spare. Can you get a spare? Is there a switch on the back perhaps, to which you are referring? I don't think so though.
Given these are relativy cheap, the coat of repair (assuming parts are available) is lily to outweigh the cost of a new one.
There's also the risk to yourself working on a microwave. These can harbour lethal voltages EVEN AFTER BEING POWERED OFF. Do you know and understand what you are doing sufficiently well to work safely?