My IS has over 170.000 km driven. As I have become a happy owner of this great car when it was 150.000km "old", I don't have a clue whether the spark plug have been replaced or not. Also I wasn't sure about the condition.
This is not any how-to, but at least you cas see how to access them.
First you must remove both parts of airbox. the each hold 3 screws.
Start with the front one:
then remove those 3 screws and pull out the main part of airbox. Prior to this, I removed the air filter. There is it is:
Next step is pretty straight forward, as you can see on following images.
overal view at the "naked" top:
remove the cable, unscrew and pull out:
as you can see, there is NGK IFR6A-11 which is one of the two recommended spark plugs (iridium only).