For the less technically minded: Android phones keep two operating systems (OS) installed, the current and the previous, which allows to fall back in case a new OS is unable to start (somewhat like Windows’ “System Restore”). What AnotherElk is suggesting is to trigger specifically that fallback mechanism.
That’s a standard: Every big feature/OS upgrade I can remember in the last 3 years had always its load of bugs and regressions, many of which have never been fixed (for instance my phone still decides to go pitch black dark now in the dark season so I need to carry a pocket lamp to see if I have new messages… 
)
My point is, you should always be wary of big updates. The almost-bi-monthly Security Updates are safe, they are mostly made by Google, but Fairphone’s feature and OS updates are dangerous (Well, this would be the last OS update for the FP4, so it had to go with a bang I guess).