two = )
Take off the fenders, then remove the gas tank. You will see the spark plug wires right there on top of the engine, remove those and voila!
The Spark plugs on the Ex250 are under the fuel tank, to change them, you'll need to remove it.
4st
There are two, you will need to remove the tank to reach them on top of the cylinder head cover.
NGK part # CR8HSA or Nippon Denso part # U24FSR-U