Change period dates

How can you take off a period? I'm three days late BUT I am never regular anyways so I'm waiting to take a test. Anyways the app thinks I'm on it right now and I went to delete it so when I start (if I do in the next day or so) I can put it then, however, when I delete it, it just moves it to the next week. That doesn't work because then the calculation of ovulation is off.