To change the default reminder for all events within a specific category, follow the steps below:

1. Open the Calendar app on your device.
2. Select the Categories button from the top navigation menu.
3. Select the category for which you want to change the default reminder time.
4. Tap the Settings button for the given category.
5. Navigate to the Reminders menu.
6. Set the default reminder time for events in this category.
7. Tap the Save button to apply the changes.