I'm having a hard time to adjusting to the new messages format in Facebook. I want to revert to the old messages format. How do I remove it? There is no option under Facebook account settings.
Unfortunately, probably not. Even if you were able to revert, it would only last until FB rolls out the new layout and makes it mandatory. I imagine that this is already the case.
Feel free to browse the info about the new features and layout: https://www.facebook.com/help/?page=260810124034353