There's a difference between downgrading and "undoing" an upgrade.
If you need to undo an upgrade, you revert to the backup you took before starting the upgrade. That's it. Done.
The other scenario is someone uses XF2 for months, acquires new users, posts, etc but then decides they want to downgrade to XF1. That's not possible to do but as Chris points out there's a good reason it's not possible.