To change the first day of your cycle, go to the "wrong" day that you have marked as your First Day, scroll down to where it says "Menstruation" and tap the highlighted menstruation data to unselect it. That day will no longer be your first day. Then, go to the actual first day of your cycle, scroll down to where it says "Menstruation" and tap the quantity of menstrual blood you observed on the first day of your period (light, medium, or heavy). That will then be the first day of your cycle.
If that doesn't work, follow the steps below:
- Go to settings and enable "start cycles manually"
- Go to data screen where you have started on the incorrect day uncheck "start new cycle today" on the data screen
- Go back to settings and re-set your cycle starts to auto.