no real easy way. I used brute force and bent my old one off over the pin. it didn't wreak the old pedal either. some guides i have read say about removing the pin, but **** thats a spring pin! once its in there theres no easy way that thing is coming out! especially not in such a confined space!
you may not want to do what i did in fear of wreaking your pedals, but basically, grab the bottom of the pedal, (hold the accelerator stalk firmly) and firmly crank the pedal toward the door. because its plastic, it should bend over the pin. then, get your replacement pedal and bend and lightly force it over the pin.
its harder to explain than it is to do. but you can do it in under 10 minutes with a bit of patience!
just be careful you dont mutilate your new pedal on the pin or you will render it useless! also take note of how the "auto correction" spring comes off! (dont fret this is easy to configure anyway!)
alternativly you might take it to holden and im sure they'd do it for you, but really they have more important things to do than fit pedals for you, so i'd say you'd get slapped with a hefty "nuience tax" to do the job.
if you have more trouble let me know and i'll try to explain it a bit better or provide pics or something...