Here are the steps that show you how to change a calendar event alert time.
By default the calendar would raise an alert 15 minutes prior to an event happening. If you need to extend the alert time to 30 minutes or an hour then follow the below steps.
1. Open up the Calendar app.
2. Then tap on an invitation on the calendar event.
3. From the Event screen tap on Alerts.
4. Now select a desire alert time.
Now you have learnt how to modify an alert time of a calendar event.