Period question for this app

Joanna • I am 22 years old, nursing student, and so in love with life and it`s twisted ways. Peace, love, and light guys! ✨
My app says I'm supposed to get my period on Monday, by the information i put down, BUT, and I hope I don't get my period, if I get AF earlier or days later than what my app says, HOW do I change it? Please help.