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 should 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), and that will then be the first day of your cycle.
If that doesn't work, follow the steps below:
Tap More in the lower right
Tap New Cycles
Tap the Menu icon in the upper left
Find the row that says "Automatic Cycle Start"
Make sure that the box is NOT checked
Once that step is complete, in the Data entry page (pencil icon) under the Menstruation section, you'll see "Start New Cycle Today?" (iPhone) or "Start New Cycle" (Android). Tap the toggle in this row to set that day to Day 1 of your cycle. You can also tap the toggle again to remove it and continue charting in the previous cycle.
Now you can go to your Chart view, then double-tap on the first day of menstruation, and tap the Start New Cycle Today? toggle to create a new cycle.