Refer to this link, not everything is the same for us 3.1+ users, so there's some things I want to point out:
- I recommend you all to delete all the pattern, fingerprints and/or passcode locks before you do all this.
- Follow the insturctions provided in the link, until this part:
- After you reboot for the first time, the system will comtinue to promnt you "com.android.media" or "com.android.phone" is not responding. Don't panic, as the phone should be booting with no problem, just touch the blank spaces to skip the dialog boxes.
- Go to phone settings, and perform factory reset.
- Wait for a while and Boom you're back on Android 9.
Disclaimer: Know what you're doing if you're going to do this. Or else, seek professional help from Nokia, but goodluck on that. I'm not responsible if you bricked your phone.