This page of the Stanley Parable Ultra Deluxe guide explains how to unlock the new content.
With the release of the Ultra Deluxe edition, Stanley Parable received new content. To access it, you'll need to get several endings from the base game. From behind each of the doors, you'll need to unlock some endings.
Once you've met the criteria, a new entrance will appear in front of the room with the two doors, with the words "NEW CONTENT" written on it. Head inside to embark on new adventures.
It's worth mentioning that you can access some of the new, hidden endings even before you get to this point.