They should just pop off. I've not got mine to hand but if I remember correctly it's held on by those plastic plugs designed to be popped back on if they come off. Just give them a slight jiggle twist back and forth while pulling lightly. Of course I accept no responsibilty if you break anything though.

I used carefully pop mine off while transforming and swap the parts over so the back kibble didn't get in the way of itself so much in robot mode.