You can change default settings for the Calendar app to make it easier to create new events and to display your calendar in a way that works best for you. For example, you can choose the default calendar for new events, change the duration of new events, and more.
If you have multiple calendars, you can choose a default calendar, and any new events you create are added to that calendar automatically.
Go to Settings > Calendar > Default Calendar.
Select the calendar you want to use as your default calendar.
Note: You can always move an event to a different calendar after you create it. To find out more about using multiple calendars, see Set up multiple calendars on iPhone.
> Calendar > Default Calendar.