How do I change DigiCal to the default calendar app?

Whenever you perform an action that requires you to open a calendar app, you will get a popup saying:

Complete action using:

1. Google Calendar

2. DigiCal

3. X

Select DigiCal and select Always

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When you have already chosen another calendar app as your default calendar and you want to change that:

Go to Settings --> Apps --> Go to the specific calendar app ---> Select clear default