See here …
Assuming you have a backup of your important data, you could try a factory reset …
https://support.fairphone.com/hc/en-us/articles/115001610926-How-to-return-to-Factory-settings-on-my-Fairphone-2
… or you could go for the clean approach of installing the new Android version …
The next update is in beta testing currently. If you want to join the testing, here’s how to sign up.