To change the default reminder time for all events in a Microsoft calendar, please follow these steps:

1. Open Microsoft Outlook.
2. Click on the ‘Settings’ icon in the upper right corner.
3. Navigate to ‘Options.’
4. Under ‘Calendar,’ click on ‘Reminders.’
5. Adjust the default time for all of the reminder options to the desired value.
6. Click on ‘Save’ to save the changes.