In short, fail to input Day 17's Password right.
Are you doing Tyson's route by chance? You can't get to the new bad timeline with him if so, you'll stop at day 20 A/B with this update.
If you're on someone else's and can't reach day 21 path F/G, outside of redownloading the update or trying a new save file I don't know. Not inputting the password on day 17 should always lead to it.