To users who have installed this internal build, please share your experience report here if you're willing to, thanks.If you want to repost this internal OTA to other websites (other than XDA itself), please ask me for permission, thanks.
If HMD will push Android 10 officially, you can install it under ADB sideload easily and link will be posted here.
Internal build version: A1N-5060-0-00WW-B01
It will be updated from: A1N-414K-0-00WW-B01, which is unreleased January 2020 Security Update.
Let me tell you how to verify:
1. Dial *#*#227#*#* to check current Version. If older than A1N-414J-0-00WW-B03, update to 414J before proceed. (Alternately, dial *#*#7837#*#* to check, proceed if displays 00WW_4_14J-B03)
FYI, 414J is December 2019 Security Update.
Alternately, you can use command "fastboot oem getversions" under fastboot mode / Download mode to check current firmware version.
2. Install A1N-414K-0-00WW-B01-414J-0-00WW-B03-update.zip with adb sideload under recovery.
3. Reboot to normal OS and recovery again - it's not over yet.
4. Install A1N-5060-0-00WW-B01-414K-0-00WW-B01-update.zip with adb sideload under recovery.
5. Reboot your phone.
If Android 10 official update pushed publicly, link will be updated here.
To prevent throttle error, I didn't upload them to Google Drive or other cloud storages. Sorry for inconvenience.