[HTC U Ultra Guides, News, & Discussion] lapaho:
Our devices have been updated to Oreo by HTC, that was a good new since there are many reports and claims from the End Users that was an unstable update, so how to downgrade to a stable Nougat version, here is the way:

[IMPORTANT]

TESTED ON HTC U Ultra DUAL SIM (INDIA - SEA VERSION) and S-ON DEVICE ONLY with UNLOCKED BOOTLOADER, BUT THIS MIGHT WORK ON OTHER VARIANTS

I. PREPARATION

1. HTC Sync Manager for drivers: http://www.htc.com/us/software/htc-sync-manager/

2. ADB and Fastboot, u can use the latest version of Minimal ADB and Fastboot:

https://xdaforums.com/showthread.php?t=2317790 (thanks to [MENTION=3558630]shimp208[/MENTION] for this great tool)

3. Stock Nougat RUU:

https://androidfilehost.com/?a=show&w=files&flid=164315 (thanks to [MENTION=1722162]sardroid[/MENTION] for RUUs uploading)

or for the tested device, this RUU has been used:

https://androidfilehost.com/?fid=962187416754465286 (thanks to [MENTION=6222398]minhnewpro[/MENTION] for uploading this)

or the latest 1.64.400.10 for Dual SEA - India I have just found

https://vnrom.net/2018/03/rom-stock-ruu-zip-cho-htc-u-ultra-ocean-note/ (original link to support to the RUU publisher)

4. Latest version of TWRP recovery:

https://twrp.me/htc/htcuultra.html (thanks Team Win for this popular and useful TWRP)

5. The most important, HTC Version Tool:

https://drive.google.com/file/d/1ubjDGNqCfIply-aRN8C7IQNb-YVkD4M7/view (thanks to [MENTION=4375068]sabpprook[/MENTION] for this super super great tool ^^ )

II. DOWNGRADING TO NOUGAT

1. Install HTC Sync Manager, ADB and Fastboot tool, extract HTC_Version_Tool.exe and copy it and twrp to your adb and fastboot folder

2. If your device's bootloader is unlocked, ignore this, if not

- go to https://www.htcdev.com/

- register for an account and then follow the site's instruction to unlock bootloader

3. Reboot your device into download mode and flash TWRP by opening cmd at your adb and fastboot folder and type:

fastboot devices
fastboot flash recovery twrp-3.2.1-1-oce.img

4. Boot device into Bootloader mode then boot to TWRP recovery

5. While your phone entered TWRP, open cmd, type:

adb devices

to check if your device is recognized

6. Once your device is recognized, open HTC_Version_Tool.exe

7. Type the version of the RUU that you are going to flash at new version line, for example, mine is 1.64.400.3 and press enter

8. Rename the RUU file to 2PZCIMG and REMEMBER TO KEEP .zip EXTENSION, then copy it to the root folder of SD Card (External SD Card is recommended)

9. Reboot your device into download mode, press Vol+, then press Power to start flashing 7.0 RUU

10. After flashing, your device can boot into Android but no signal or IMEI received, dont worry, continue these steps

11. Reboot into download mode, reflash TWRP, then boot to TWRP

12. At cmd windows, type

adb devices
adb shell
mke2fs -t ext4 /dev/block/bootdevice/by-name/persist

13. Reboot device to check if IMEI received

14. If IMEI received, you should reflash stock recovery from RUU.zip, but reflash the RUU is recommended.

If not, try to format the persist partition manually again (step 12)

15. Done

And at last, I want to say big thanks to [MENTION=4375068]sabpprook[/MENTION] for helping me a lot to downgrade the device and [MENTION=8789761]paulpier007[/MENTION] for HTC U11 downgrade method which gave me the idea for downgrading our U Ultra (https://xdaforums.com/u11/how-to/guide-downgrade-htc-u11-oreo-to-nougat-t3754481)