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.
This is a tough question as most Filipino women would not leave their country on a K1 visa and move to the US to be married as it would be out of tradition. When they leave the house of their parents it is respectful to leave as a married woman and not as a fiancee. The problem is the US visa and how best to move around this.