A purchase can't actually be cancelled from the buyer's end, as such, it has to be initiated at the seller's end. If they are agreeable to cancelling the purchase, you can ask they they send you a mutual cancellation request.
Any other action that could possibly be taken would depend on the specifics of your particular transaction, the item in question, how it was misrepresented and so forth.
Have you paid for it, or received it, or is there something else that's come to light which indicates the item has been misrepresented?
