Hi and welcome to the forum.
You can revert by manually installing the previous update.
Here is a link to dowload an image of the previous version 0013.4
https://fairphone-android-builds.ams3.digitaloceanspaces.com/FP3/FP3-REL-8901.4.A.0013.4-gms-c1b5afb7-user-fastbootimage.zip
Here is the page for links to all versions
Here are official instructions on how to manually install a version.
https://support.fairphone.com/hc/en-us/articles/360048050332-FP3-Install-Fairphone-OS-offline
Before you do this I would recommend you contact Fairphone urgently either via email support@fairphone.com or by calling
For urgent/existing requests you can contact us by phone:
+31 20 788 4400
Monday to Friday, 10:00-16:00 CET