Since the transaction already has confirmed, it can't be reversed/replaced/manipulated in any way.
The only way to get your money back is to contact the support and describe your problem. They have to either send you the money back
or to fullfill your order. But it may be the case tha the vendor "choses" to fullfill the order even if you don't want to anymore. Its mostly up to him,
since you agreed to his terms by ordering something. But for the next time ordering i would suggest to use
https://bitcoinfees.earn.com/to get a fee which will get the transaction through within a given time period. I'd rather pay 0.50$ more for TX fee than having the hassle of getting my money back.