Outlook Web App provider users with the option for changing the calendar reminders. Using these options users can turn on or turn off the display of reminder, reminder sound and also change the default reminder time. You can modify Outlook Web App Calendar Reminders using Calendar settings under Outlook Options.
Login to Outlook Web Apps in Office 365 and then click on the Options link
In the Outlook Web App Options screen, navigate to Reminder section under Calendar Settings
You can turn on the reminders by marking the check box with label as Show reminder alerts.
Similarly if you want to enable the reminder sound then mark the check box with label as Play a sound when a reminder is due. And to you can set your desired reminder time in Default reminder dropdown. After making the required changes, click on the Save link on the bottom of the screen to confirm and save the changes.