Falling in and out of love in the middle of a visa process can become complicated. If you have decided not to get married and wish to cancel the K1 visa then you would need to contact the US embassy in the country where she lives before the interview date.
You would have to provide them proof that you are the person in question and provide copies of the Notice of Action letters to verify that it is you and then explain to them in a letter why and what has happened this then. You can then request to cancel the interview at the embassy.
1. They need to identify you so an email or fax will not do;
2. You need to write a letter which has been notarized to request cancellation;
3. You also need to forward a copy to the USCIS in your area.
This normally takes care of it however don’t assume that it has been cancelled until such time that they reply with confirmation that it has been cancelled. Don’t make use of the mail but try and make use of a courier service to ensure that the mail gets there as soon as possible.