how do you change a key switch on a 73 nova?

Answer 1

If you mean the ignition cylinder where you put the key in, you have to disassemble the steering column to remove the cylinder...buy a Chilton or Haynes service manual for your car at a local parts store or try Eautorepair.com online for 10.00 to get a procedure for doing this. This link shows a 70 Chevelle column being done which is similiar:
http://www.carcraft.com/howto/56459/ just follow the steps to the point where they remove the cylinder and then reverse the order to reinstall.
You'll have to buy some special tools to do it also such as a lockplate compressor and steering wheel puller which you can find at most parts stores and sometimes they can be rented. Hope this helps, Paul

Answer 2

If the key switch is in the steering column then make sure you take it to someone that knows what they are doing.
It would involve disassembling the column to get to the key switch. It should take about an hour to do.