According to Facebook’s Help Centre page, the company has stated that users could only switch back to the classic look of Facebook temporarily until the end of August.
Starting in September, users will be unable to make a switch back to the older UI as the feature of switching back will be unavailable. Facebook users will have to adapt to the new look that has already been made available for use.
Should Facebook users wish to make a switch from the old to the new UI, they can continue to do so by clicking on the “Down” arrow menu button situated along the Facebook search bar area and toggle the “Switch to new Facebook” option that will be found within the same list. While the new major update on Facebook will provide a completely new look and feel for the social media platform, users might want to consider switching to the new UI now in order to get themselves familiar with the updated change. (Source: Engadget)