to remove everything iocage related from the currently activated pool and then run iocage activate <pool of choice> to get it to activate on the pool of your choice.
As far as I understand it, cleaning will remove everything including any datasets for jail already created.
I wish I'd know this when I was setting it up originally as I deleted everything manually and it was a PITA. Note that I've not tried the first command myself and that you should read the man page to ensure you understand what you are doing first. Clearly I didn't take my own advice.