How do i undo the upgrade?

The Calendar app is part of the operating system (iOS). When you update the iOS then you automatically update the Calendar app. Apple does NOT support downgrading of an iOS version.

If you believe that there are some operations of the Calendar app require changes then please make suggestions directly to Apple by leaving a product feedback here ➔ Product Feedback - Apple 

Axel F.

You can’t undo an upgrade.